A Free Syrian Army fighter looks through a hole from behind sandbags, while a fellow fighter reads the Quran in Deir Ezzor, Syria, on Tuesday, September 3. The United States and other Western nations blame the Assad regime for a chemical weapons attack that's believed to have killed more than 1,400 people. Tensions in Syria began to flare in March 2011 and escalated into an ongoing civil war. Click through to view the most compelling images taken since the start of the conflict: 
A Free Syrian Army fighter takes cover as he watches forces loyal to President Bashar al-Assad near Hanano barracks in Aleppo on September 3. 
A Free Syrian Army fighter is seen through a hole in a wall of a building in Aleppo on September 3. 
Free Syrian Army fighters walk inside a damaged house in Aleppo on September 3. 
A Free Syrian Army fighter walks through a hole in a wall in Aleppo on September 3. 
Free Syrian Army fighters talk inside a burnt house in Aleppo on September 3. 
A Free Syrian Army fighter peeks through the curtains of a bedroom in Deir ez-Zor, Syria, on Monday, September 2. 
Free Syrian Army fighters take their positions behind piled sandbags, as one of them points his weapon, in Deir ez-Zor on September 2. 
A rebel fighter points his weapon at Syrian regime forces in Deir ez-Zor on September 2. 
Syrian firefighters try to extinguish a fire after a missile hit a residential building in Damascus, Syria, on September 2. 
A U.N. arms expert collects samples during an inspection of a suspected chemical weapons strike site in the Ghouta area outside Damascus on August 29. 
People search for belongings in rubble in Raqqa, Syria, on August 29. 
Free Syrian Army fighters launch a rocket toward forces loyal to Syrian President Bashar al-Assad in Deir Ezzor on August 29. 
A Syrian Kurd uses hay to hide another woman in a training session organized by the Kurdish Women's Defense Units on Wednesday, August 28, in a northern Syrian border village. They're preparing if the area comes under attack. 
Free Syrian Army fighters escort U.N. vehicles with chemical weapons experts on August 28 through a site of a suspected chemical weapons attack outside Damascus. 
A Free Syrian Army fighter takes position behind sandbags in the old city of Aleppo, Syria, on Tuesday, August 27. 
A U.N. team leaves its Damascus, Syria, hotel in a convoy on Monday, August 26. The team was to investigate an alleged chemical attack that killed hundreds last week in a suburb of the Syrian capital. Sniper fire hit a vehicle used by the U.N. chemical weapons investigation team multiple times Monday, according to the United Nations. 
A Syrian soldier walks down a street in Damascus on Saturday, August 24. 
Pigeons lie dead on the ground on August 24 from after what activists say is the use of chemical weapons by government forces in the Damascus suburb of Arbeen. 
Columns of smoke rise in Barzeh after heavy shelling on Friday, August 23. 
A young Free Syrian Army fighter is reflected in a mirror as he takes position in a house in Aleppo on Thursday, August 22. 
Rebels move around a building in Aleppo on August 22. 
Syrian rebels claim pro-government forces used chemical weapons to kill citizens outside Damascus on Wednesday, August 21. People inspect bodies in this photo released by the Syrian opposition Shaam News Network. 
People search the rubble of a bombed building in Aleppo, Syria, on Friday, August 16. 
Men bury the bodies of six members of the same family killed in a bombing in Raqqa on Saturday, August 10. 
Syrian Army soldiers patrol a devastated street in Homs on Wednesday, July 31. 
Free Syrian Army fighters move through a hole in a wall in Khan al-Assal on Monday, July 22, after seizing the town. 
A rebel fighter walks past swings in a deserted playground in Deir al-Zor, Syria, on Sunday, July 21. 
A rebel fighter speaks with a fellow fighter through a hole in a wall in Deir al-Zor on July 21. 
A Free Syrian Army fighter casts a shadow on a wall as he carries his weapon in a shelter in Deir al-Zor on Thursday, July 18. 
Yahya Sweed, 13, is comforted by his father as he lies on a bed in Kfar Nubul on Tuesday, July 16. The boy was injured by shrapnel, resulting in the amputation of his right leg. 
A rebel fighter naps in a trench about 300 feet from the Syrian government forces' positions along the highway connecting Idlib with Latakia on Monday, July 15. 
A rebel fighter uses a hole in the wall of a destroyed school to aim at Syrian government forces in the Izaa district of Aleppo on Sunday, July 14. 
A Free Syrian Army fighter uses a mirror to scope out snipers loyal to Syrian President Bashar al-Assad in Aleppo on Friday, July 12. 
A Free Syrian Army fighter stands over a boy who was injured during shelling in Al-Bara on Monday, July 8. 
Members of the Free Syrian Army fire a homemade rocket toward regime forces in Deir al-Zor on Sunday, June 16. 
Syrian rebels leave their position in the northwestern town of Maaret al-Numan on Thursday, June 13. The White House said that the Syrian government has crossed a "red line" with its use of chemical weapons and announced it would start arming the rebels. 
Forces loyal to Syrian President Bashar al-Assad are seen near Qusayr on Thursday, May 30. 
Syrian rebels take position in a house during clashes with regime forces in the old city of Aleppo on May 22. 
Syrian army soldiers take control of the village of Western Dumayna north of the rebel-held city of Qusayr on Monday, May 13. Syrian troops captured three villages in Homs province, allowing them to cut supply lines to rebels inside Qusayr town, a military officer told AFP. 
Rebel fighters fire at government forces in the northern Syrian city of Aleppo on Sunday, May 12. 
Searchers use a flashlight as they look for survivors among the rubble created by what activists say was a missile attack from the Syrian regime, in Raqqa province, Syria, on April 25. 
A Kurdish fighter from the "Popular Protection Units" (YPG) takes position inside a building in the majority-Kurdish Sheikh Maqsood area of Aleppo, on Apri. 21. 
People walk past destroyed houses in the northern Syrian town of Azaz on Sunday, April 21. 
Free Syrian Army fighters take positions prior to an offensive against government forces in the Khan al-Assal area, near Aleppo on Saturday, April 20. 
Men inspect damage at a house destroyed in an airstrike in Aleppo on April 15. 
Syrian and Kurdish rebel fighters walk in the Sheikh Maqsud district of Aleppo on April 14. 
A female rebel monitors the movement of Syrian government forces in Aleppo's Sheikh Maqsud neighborhood on Thursday, April 11. 
A rebel runs to avoid sniper fire from Syrian government forces in Aleppo on April 11. 
Syrian rebels observe the movement of Syrian government forces around Al-Kendi hospital in Aleppo on Wednesday, April 10. 
Rescue teams and security forces check out the scene of a deadly car bomb explosion in Damascus on April 8. 
The fighting has taken a toll on buildings in Aleppo's Saladin district, seen here on April 8. 
A Syrian rebel runs for cover in Deir ez-Zor on April 2. 
A rebel checks for snipers across the street toward the Citadel in Aleppo, Syria, on Saturday, March 30, in this photo taken by iReporter Lee Harper. 
A Free Syrian fighter mourns the death of a friend in Aleppo on March 30, in this photo taken by iReporter Lee Harper. 
A Syrian opposition fighter runs for cover from Syrian army snipers in Aleppo on Wednesday, March 27. 
A Syrian girl covers her face to protect herself from fumes as a street covered with uncollected garbage is fumigated in Aleppo on Sunday, March 24. 
A Syrian man and his family drive past damaged buildings in Maarat al-Numan, on Wednesday, March 20. 
Syrians carry the body of a Syrian army soldier during a funeral ceremony in Idlib province on Tuesday, March 19. 
Syrian rebels take position in Aleppo, the largest city in the country, on March 11. 
Syrian men search for their relatives amongst the bodies of civilians executed and dumped in the Quweiq River on March 11. 
A Free Syrian Army fighter looks back as smoke rises during fighting between rebel fighters and forces loyal to Syria's President Bashar al-Assad on the outskirts of Aleppo on Saturday, March 2. 
Residents read Shaam News newspapers published by the Free Syrian Army in Aleppo on March 2. 
A member of the Free Syrian Army reacts to the death of a comrade who was killed in fighting, at Bustan al Qasr cemetery in Aleppo on Friday, March 1. 
A rebel fighter throws a home-made grenade at Syrian government forces in Aleppo on February 16. 
A member of the Free Syrian Army stands with his weapon as he looks at a rainbow in Aleppo on February 16. 
A Syrian woman looks through a bus window in Aleppo on February 14. 
Free Syrian Army fighters walk through a dust-filled stairwell in Damascus on February 7. 
A Syrian rebel gestures at comrades from inside a broken armored personnel carrier in Al-Yaqubia on February 6. 
A rebel fighter throws a hand grenade inside a Syrian Army base in Damascus on February 3. 
People stand in the dust of a building destroyed in an airstrike in Aleppo, Syria on February 3. 
Free Syrian Army fighters run as they enter a Syrian Army base during heavy fighting in the Arabeen neighborhood of Damascus on February 3. 
An unexploded mortar shell fired by the Syrian Army sits lodged in the ground in Damascus on January 25. 
Fighters from Fateh al Sham unit of the Free Syrian Army fire on Syrian Army soldiers at a check point in Damascus on January 20. 
A Free Syrian Army fighter walks between buildings damaged during Syrian Air Force strikes in Damascus on January 19. 
A Syrian rebel fighter tries to locate a government jet fighter in Aleppo on January 18. 
Syrian rebels launch a missile near the Abu Baker brigade in Albab on January 16. 
A Syrian boy walks near rubbish next to tents at a refugee camp near the northern city of Azaz on the Syria-Turkey border, on January 8. 
Syrians look for survivors amid the rubble of a building targeted by a missile in Aleppo on January 7. 
A father reacts after hearing of a shelling by forces loyal to Syria's President Bashar al-Assad in Aleppo on January 3. 
A patient smokes a cigarette at Dar Al-Ajaza psychiatric hospital in Aleppo on December 18, 2012. The psychiatric ward, housing around 60 patients, has lacked the means to function properly since fighting broke out there in July. 
Syrians mourn a fallen rebel fighter at a rebel base in the al-Fardos area of Aleppo on December 8. 
Members of Liwa (Brigade) Salahadin, a Kurdish military unit fighting alongside rebel fighters, monitor the area in the besieged district of Karmel al-Jabl in Aleppo on December 6. 
A member of Liwa Salahadin aims at a regime fighter in the besieged district of Karmel al-Jabl in Aleppo on December 6. 
Two young boys sit underneath a washline in a refugee camp on the border between Syria and Turkey near Azaz on December 5. 
The bodies of three children, who were allegedly killed in a mortar shell attack that landed close to a bakery in Aleppo, on December 2, are laid out for identification by family members at a makeshift hospital at an undisclosed location of the city. 
Smoke rises in the Hanano and Bustan al-Basha districts in Aleppo on December 1 as fighting continues through the night. 
Damaged houses in Aleppo are seen after an airstrike on November 29. 
A Syrian rebel mourns the death of a comrade in Maraat al-Numan on November 20. 
Syrians protesters stand on Assad's portrait during an anti-regime demonstration in Aleppo on November 16. 
A Syrian rebel takes cover during fighting against Syrian government forces in Aleppo on November 15. 
Syrian opposition fighter Bazel Araj, 19, sleeps next to his pistol in Aleppo on November 11. 
A rebel fighter fires at a Syrian government position in Aleppo on November 6. 
A Syrian rebel leaps over debris left in the street while running across a "sniper alley" near the Salahudeen district in Aleppo on November 4. 
Rebels hold their position in the midst of a battle on November 3 in Aleppo. 
A man cries while being treated in a local hospital in a rebel-controlled area of Aleppo on October 31. 
A man is treated for wounds after a government jet attacked the Karm al-Aser neighborhood in eastern Aleppo on October 31. 
A Syrian rebel interrogates a handcuffed and blindfolded man suspected of being a pro-regime militiaman in Aleppo on October 26. 
Smoke rises from a fuel station following a mortar attack as Syrian women walk on a rainy day in the Arqub neighborhood of Aleppo on October 25. 
A Syrian rebel fires at an army position in the Karm al-Jabal district of Aleppo on October 22. 
A wounded Syrian boy sits on the back of a truck carrying victims and wounded people to a hospital following an attack by regime forces in Aleppo on October 21. 
A man lies on the ground after being shot by a sniper for a second time as he waits to be rescued by members of the Al-Baraa Bin Malek Battalion, part of the Free Syria Army's Al-Fatah brigade, in Aleppo on October 20. 
Syrian army soldiers run for cover during clashes with rebel fighters at Karam al-Jabal neighborhood of Aleppo on October 20. 
Smoke rises after a Syrian Air Force fighter jet fired missiles at the suburbs of the northern province of Idlib on October 16. 
A Syrian opposition fighter stands near a post in Aleppo on October 11. 
A Syrian man mourns the death of his father, who was killed during a government attack in Aleppo on October 10. 
A rebel fighter is carried by his friends and laid on a gurney to be treated for gunshot wounds sustained during heavy battles with government forces in Aleppo on October 1. 
Syrian rebels help a wounded comrade to an Aleppo hospital after he was injured in a Syrian army strike on September 18. 
Free Syria Army fighters are reflected in a mirror they use to see a Syrian Army post only 50 meters away in Aleppo on September 16. 
A Syrian man carrying grocery bags tries to dodge sniper fire as he runs through an alley near a checkpoint manned by the Free Syria Army in Aleppo on September 14. 
A woman walks past a destroyed building in Aleppo on September 13. 
Free Syrian Army fighters battle during street fighting against Syrian army soldiers in Aleppo on September 8. 
A Syrian man wounded by shelling sits on a chair outside a closed shop in Aleppo on September 4. 
A woman sits in her wheelchair next to her house, damaged by a Syrian air raid, near Homs on August 26. 
Members of the Free Syrian Army clash with Syrian army soliders in Aleppo's Saif al-Dawla district on August 22. 
A man mourns in front of a field hospital on August 21 in Aleppo. 
Wounded civilians wait in a field hospital after an air strike on August 21 in Aleppo. 
People pray during the funeral of a Free Syrian Army fighter, Amar Ali Amero, on August 21. 
A man cries near the graves of his two children killed during a recent Syrian airstrike in Azaz on August 20. 
A Syrian woman holds her dead baby as she screams upon seeing her husband's body being covered following an airstrike by regime forces on the town of Azaz on August 15. 
A Syrian rebel runs in a street of Selehattin during an attack on the municipal building on July 23. 
Syrian rebels hunt for snipers after attacking the municipality building in the city center of Selehattin on July 23. 
Members of the Free Syrian Army's Mugaweer (commandos) Brigade pay their respects in a cemetery on May 12 in Qusayr. 
Syrian rebels take position near Qusayr on May 10. 
A Free Syrian Army member takes cover in underground caves in Sarmin on April 9. 
Rebels prepare to engage government tanks that advanced into Saraquib on April 9. 
Men say prayers during a ceremony in Binnish on April 9. 
A young boy plays with a toy gun in Binnish on April 9. 
A Free Syrian Army rebel mounts his horse in the Al-Shatouria village near the Turkish border in northwestern Syria on March 16, a year after the uprising began. 
Syrian refugees walk across a field before crossing into Turkey on March 14. 
A rebel takes position in Al-Qsair on January 27. 
A protester in Homs throws a tear gas bomb back towards security forces, on December 27, 2011. 
A man stands under a giant Syrian flag outside the Umayyad Mosque in Damascus on December 24, 2011. 
A member of the Free Syrian Army looks out over a valley in the village of Ain al-Baida on December 15, 2011. 
Members of the Free Syrian Army stand in an valley near the village of Ain al-Baida, close to the Turkish border, on December 15, 2011. 
Displaced Syrian refugees walk through an orchard adjacent to Syria's northern border with Turkey on June 14, 2011, near Khirbet al-Jouz. 
A Syrian man holds up a portrait of President Bashar al-Assad during a rally to show support for the president in Damascus on April 30, 2011. 
Syrians rally to show their support for President Bashar al-Assad in Damascus on April 30, 2011. 
A screen grab from YouTube shows thick smoke rising above as Syrian anti-government protesters demonstrate in Moaret Al-Noman on April 29, 2011. 
A screen grab from YouTube shows Syrian anti-government protesters run for cover from tear gas fired by security forces in Damascus on April 29, 2011, during the "Day of Rage" demonstrations called by activists to put pressure on al-Assad. 
Syrians wave their national flag and hold portraits of al-Assad during a rally to show their support for their leader in Damascus on March 29, 2011. 
A woman sits by the hospital bed of a man allegedly injured when an armed group seized rooftops in Latakia on March 27, 2011, and opened fire at passers-by, citizens and security forces personnel according to official sources. 
Syrian protesters chant slogans in support of al-Assad during a rally in Damascus on March 25, 2011. 
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- Kerry says the U.S. has declassified "unprecedented amounts of information"
- The U.S. points to "concrete" evidence, intercepts, "multiple streams of information"
- Britain, France, and Germany come to similar conclusions
- Putin calls for "a deep and specific probe containing evidence that would be obvious"
(CNN) -- U.S. Secretary of State John Kerry calls it proof "beyond any reasonable doubt." Defense Secretary Chuck Hagel says it's "very clear."
President Barack Obama says the United States has "high confidence" that Syria used chemical weapons -- the strongest position the U.S. can take short of confirmation.
Britain, France, and Germany say their intelligence backs up the same conclusion.
But despite all the talk about conclusive intelligence, questions remain. A declassified report by the White House does not divulge all details of the evidence the United States is looking at. And it remains unclear what the "streams of intelligence" cited in the report may be and how they were collected.
Russia insists there's no proof. Russian President Vladimir Putin said he wants to see evidence that would make the determination "obvious."
Syria: Searching for solutions 
Kerry: Riskier to not act against Syria 
Hagel: 'U.S. forces ready to act' And Syrian President Bashar al-Assad's regime says rebel forces were behind any chemical weapons attack.
The United Nations, meanwhile, is calling on world leaders to wait for the results of a U.N. probe.
Here's a look at what is known about the intelligence Washington points to.
Kerry cites 'concrete' evidence
"We have declassified unprecedented amounts of information, and we ask the American people and the rest of the world to judge that information," Kerry told lawmakers Tuesday.
It "proves the Assad regime prepared for this attack, issued instruction to prepare for this attack, warned its own forces to use gas masks."
Physical, "concrete" evidence shows where the rockets came from, when they were fired, and that not one landed in regime-controlled territory, Kerry said.
"Multiple streams of intelligence indicate that the regime executed a rocket and artillery attack against the Damascus suburbs in the early hours of August 21," the White House says in the declassified report.
"Satellite detections corroborate that attacks from a regime-controlled area struck neighborhoods where the chemical attacks reportedly occurred. ... The lack of flight activity or missile launches also leads us to conclude that the regime used rockets in the attack."
The White House released a map along with the report.
But the prospect of more details on the evidence seems dim as Kerry said the amount already declassified "could possibly put at risk some sources and methods," and that releasing more could "tempt fate."
U.S.: Opposition doesn't have 'the capacity'
"We are certain that none of the opposition has the weapons or capacity to effect a strike of this scale, particularly from the heart of regime territory," Kerry told lawmakers Tuesday.
Kerry: Assad behind 'outrageous attack' 
Senator: 'Embarrassed' on Syria reaction 
Syria crisis 'is not our problem' The White House report points to Syria's known stockpiles of chemical agents. And it says the United States assesses "with high confidence that the Syrian regime has used chemical weapons on a small scale against the opposition multiple times in the last year, including in the Damascus suburbs. This assessment is based on multiple streams of information including reporting of Syrian officials planning and executing chemical weapons attacks and laboratory analysis of physiological samples obtained from a number of individuals, which revealed exposure to sarin.
"We assess that the opposition has not used chemical weapons."
In May, a U.N. official said there were strong suspicions that Syrian rebel forces had used sarin gas. But the findings were not conclusive, the U.N. Independent International Commission of Inquiry for Syria said at the time, and the opposition Syrian Coalition condemned any use of chemical weapons. The U.S. State Department said at the time it had no evidence suggesting rebels had used chemical weapons.
The Russian Foreign Ministry said Wednesday the results of an investigation into a March attack in Aleppo, apparently using chemical weapons, found that the charge used was homemade and similar to projectiles produced by the group Bashaar al-Nasr, part of the opposition Syrian Islamic Liberation Front. Sarin was discovered in samples from the scene, the foreign ministry said.
Bashaar al-Nasr has slammed Syria for the recent chemical weapons attack in Damascus, and vowed revenge.
U.S.: Syria prepared
"In the three days prior to the attack, we collected streams of human, signals and geospatial intelligence that reveal regime activities that we assess were associated with preparations for a chemical weapons attack," the U.S. report says.
"Syrian chemical weapons personnel were operating in the Damascus suburb of 'Adra from Sunday, August 18 until early in the morning on Wednesday, August 21 near an area that the regime uses to mix chemical weapons, including sarin. On August 21, a Syrian regime element prepared for a chemical weapons attack in the Damascus area, including through the utilization of gas masks. Our intelligence sources in the Damascus area did not detect any indications in the days prior to the attack that opposition affiliates were planning to use chemical weapons."
'Intercepted communications'
"We have a body of information, including past Syrian practice, that leads us to conclude that regime officials were witting of and directed the attack on August 21," the U.S. report says. "We intercepted communications involving a senior official intimately familiar with the offensive who confirmed that chemical weapons were used by the regime on August 21 and was concerned with the U.N. inspectors obtaining evidence."
Intelligence shows Syrian chemical weapons personnel were told to cease operations in the afternoon of August 21, and that the regime then "intensified the artillery barrage targeting many of the neighborhoods where chemical attacks occurred," the report says.
Analyst: 'No way in hell' U.S. can back up death toll
The U.S. report says a preliminary assessment "determined that 1,429 people were killed in the chemical weapons attack, including at least 426 children."
The assessment "will certainly evolve as we obtain more information," it adds.
"Secretary Kerry seems to have been sandbagged into using an absurdly over-precise number," said Anthony Cordesman, former director of intelligence assessment at the U.S. Defense Department.
Now with the Center for Strategic and International Studies, he writes on the CSIS website, "Put simply, there is no way in hell the U.S. intelligence community could credibly have made an estimate this exact."
It's unclear whether "these figures really had an intelligence source," Cordesman said . "Some sources indicate they may have actually come from a Syrian source called the Local Coordination Committees (LCC)" -- a Syrian opposition group.
Analysis: Israel role in Syria debate 
Assad dynasty began with coup 
Representatives debate action in Syria A U.S. official told CNN the number is not based on opposition figures. The methodology used to come up with the toll remains classified.
Rebel leaders have given similar estimates for the death toll, saying more than 1,300 people were killed.
Britain's Joint Intelligence Organization, meanwhile, says at least 350 people were killed. It does not say how the figure was determined.
A French government report notes that body counts by several sources, including Doctors Without Borders, estimated at least 355 deaths. "Other technical counts, using different sources, estimate the toll to be around 1,500 deaths," the report says.
Britain, France, Germany weigh in
A letter from Jon Day, chairman of Britain's Joint Intelligence Committee, to British Prime Minister David Cameron, also rejects suggestions that the opposition may have been behind the attack.
"We have tested this assertion using a wide range of intelligence and open sources, and invited (the government) and outside experts to help us establish whether such a thing is possible," Day wrote.
No "credible intelligence" suggests the opposition has chemical weapons, he writes, adding that "there are no plausible alternative scenarios to regime responsibility."
"We also have a limited but growing body of intelligence which supports the judgment that the regime was responsible for the attacks and that they were conducted to help clear the opposition from strategic parts of Damascus. Some of this intelligence is highly sensitive but you have had access to it all."
France gives a similar argument.
"The attack of August 21st could only have been ordered and carried out by the regime," its declassified intelligence report says.
Germany's foreign intelligence agency, the BND, has the same assessment.
"In a secret briefing to select lawmakers on Monday, BND head Gerhard Schindler said that while there is still no incontestable proof, analysis of the evidence at hand has led his intelligence service to believe that Assad's regime is to blame," Der Spiegel reports.
U.N. probe: Limited scope, no clear deadline
The United Nations is pushing all nations to hold off on any action until results of its own examination are in. It's unclear how soon that may be.
"The U.N. mission is uniquely capable of establishing in an impartial and credible manner the facts of any use of chemical weapons," Martin Nesirky, spokesman for Secretary-General Ban Ki-moon, said repeatedly at a news conference Sunday.
But the U.N. probe's mandate is only to determine whether chemical weapons were used -- not by whom.
Obama argued Wednesday that that's no longer in question. "Frankly, nobody is really disputing that chemical weapons were used," he said.
Russia - which, along with China, would likely block any U.N. resolution authorizing military action in Syria -- has repeatedly thrown cold water on suggestions that there's proof of Syria culpability.
"If there are data that the chemical weapons have been used, and used specifically by the regular army, this evidence should be submitted to the U.N. Security Council," Putin said Wednesday in an interview with The Associated Press and Russia's state Channel 1 television.
"It should be a deep and specific probe containing evidence that would be obvious and prove beyond doubt who did it and what means were used."
If there was such evidence, Russia might support a resolution authorizing military strikes, Putin said.
Syria: Allegations 'false and unfounded'
Syria still insists it never used chemical weapons.
"These allegations are false and unfounded," Syria's U.N. Ambassador Bashar al-Ja'afari said Tuesday in an interview with CNN.
He cited the confidence that the United States said it had a decade ago when it argued that Iraq had weapons of mass destruction.
It's an argument Kerry was prepared for before Congress on Tuesday.
He noted that he and Hagel were senators at the time of the Iraq vote.
"And so we are especially sensitive, Chuck and I, to never again asking any member of Congress to take a vote on faulty intelligence. And that is why our intelligence community has scrubbed and re-scrubbed the evidence."
