A Syrian soldier walks down a street in Damascus on Saturday, August 24. Tensions in Syria began to flare in March 2011 and have escalated into an ongoing civil war. Click through to view the most compelling images taken since the start of the conflict. 
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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- Syria tells CNN it will allow inspectors access to site of attack
- U.S. official: "Only the Assad regime is responsible for chemical weapons use"
- The Syrian regime denies using chemical weapons
- Russia cites Iraq, urges U.S. to not repeat mistakes of the past
Damascus, Syria (CNN) -- As U.N. inspectors get ready to look over the site of a suspected chemical weapons attack in Syria, a U.S. official told CNN there is almost no doubt that the Bashar al-Assad regime is responsible.
"There is nothing credible to indicate that the rebels, either the Syrian National Council or even al-Nusra Front, have used chemical weapons," the official said. "Only the Assad regime is responsible for chemical weapons use."
Rebel forces and the Syrian regime have been blaming each other for Wednesday's reported attack in a suburb of Damascus, which opposition members say killed hundreds. Gruesome video of the aftermath showed numerous bodies, including women and children.
The official, who is not authorized to speak on the record, said the evidence goes beyond images and open-source reporting from doctors and others, and said there is a "wide range of tools" to collect and analyze enough data to make an informed assertion.
The official's comments came as a top Syrian official told CNN on Sunday that the government would allow U.N. inspectors full access to any site of a suspected chemical weapons attack.
The agreement is effective immediately, Deputy Foreign Minister Faisal al Mekdad said.
The U.N. secretary-general's office said inspectors hope to begin their investigation Monday at the suspected chemical attack site. Before Sunday, U.N. inspectors in Syria attempting to gather information were not allowed to visit the site of the recent attack.
Russia issues warning to U.S. over Syria 
Syria agrees to let U.N. investigate 
World reacts to alleged chemical attack 'Too little, too late'
One senior Obama administration official said the Syrian government's announcement was too little, too late.
"If the Syrian government had nothing to hide and wanted to prove to the world that it had not used chemical weapons in this incident, it would have ceased its attacks on the area and granted immediate access to the UN -- five days ago," the official said Sunday.
"At this juncture, the belated decision by the regime to grant access to the U.N. team is too late to be credible, including because the evidence available has been significantly corrupted as a result of the regime's persistent shelling and other intentional actions over the last five days," the official, who was not authorized to speak publicly, said in a statement.
"Based on the reported number of victims, reported symptoms of those who were killed or injured, witness accounts, and other facts gathered by open sources, the U.S. intelligence community, and international partners, there is very little doubt at this point that a chemical weapon was used by the Syrian regime against civilians in this incident."
U.S. 'continuing to assess the facts'
The same senior Obama administration official said the United States is "continuing to assess the facts so the president can make an informed decision about how to respond to this indiscriminate use of chemical weapons."
Last year, while discussing the situation in Syria, U.S. President Barack Obama said "a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized."
And two key members of U.S. congressional foreign affairs panels said Sunday they expect the United States to strike Syria following reports of chemical weapons attacks in that country.
On Friday, Chuck Hagel said the American military was providing President Barack Obama "with options for all contingencies, and that requires positioning our forces [and] positioning our assets to be able to carry out whatever options the president might choose."
The Pentagon has sent four warships armed with cruise missiles to the region.
Russia issues warning to U.S. over Syria 
Obama's Syria problem 
Sen. John McCain on Syria Russia: Don't repeat mistakes of the past
Russia's Ministry of Foreign Affairs urged Washington to avoid actions inconsistent with international law.
"Once again we strongly urge you not to repeat the mistakes of the past," the ministry said.
In a statement, spokesman A.K. Lukashevich said: "All this is reminiscent of events of 10 years ago in which, using false information that the Iraqis possessed weapons of mass destruction as a pretext and bypassing the United Nations, the United States launched a reckless enterprise with consequences that everyone is well aware of."
Russia said any unilateral armed action that bypassed the United Nations would "undermine international efforts to find a political and diplomatic solution to the Syrian conflict, lead to its further escalation, and have an extremely destructive effect on what already is an explosive situation in the Middle East."
The statement goes on to say: "Our American and European partners should realize what catastrophic consequences such a policy would have for the region, for the Arab world, and for the entire Islamic world."
On Sunday, an Iranian military official warned the United States against crossing Syria's own "red line."
Massoud Jazayeri, a deputy chief of staff of Iran's armed forces, said it would have "severe consequences," according to the semiofficial Fars news agency.
Jihadist group vows revenge
The jihadist group al-Nusra Front, which has ties to al Qaeda, vowed Sunday to revenge the attack by targeting Alawite villages.
Competing claims on chemical weapons use in Syria
An audio message posted on YouTube, purportedly from al-Nusra leader Abu Muhammad al-Joulani, accuses the Syrian regime of bombarding eastern Ghouta "with tens of missiles" carrying a "suffocating chemical agent," killing "hundreds of children, women and men."
"We are announcing a series of revenge operations called 'An Eye for an Eye.' Your Alawite villages will pay a very dear price for every chemical rocket that you've launched against our people."
Syrian President Bashar al-Assad is part of the country's Alawite minority.
Kerry talks with Syrian diplomat
Syrian government denial
The Syrian government has steadfastly denied its forces used chemical weapons outside Damascus or elsewhere and repeated the denial Sunday.
According to Syrian state-run television's depiction of events, government forces came across the site of the gas attack when they entered the rebel stronghold of Jobar on the edge of Damascus. The bodies of some of those killed in the attack early Wednesday had been found there.
Several of the soldiers were "suffocating" from exposure to gases as they entered the city, according to state TV.
"It is believed that the terrorists have used chemical weapons in the area," Syrian TV reported, citing an anonymous source. The government uses the term "terrorists" to describe rebel forces.
Broadcast video showed a room containing gas masks, gas canisters and other paraphernalia that could be used in a gas attack. The army said it uncovered the cache in a storage facility in Jobar. CNN could not independently confirm the authenticity of the video.
"We said it from the first moment and, here, we assure again that we have never used chemical weapons (around Jobar) or any other region in any form whatsoever -- ... liquid, gas or whatever," Information Minister Omran al-Zoubi said in an interview on Lebanese television.
He blamed the rebels.
Syria's refugee children will be the ones to rebuild
Opposition accusations
The rebels say that government forces fired rockets into the heavily populated civilian area. Opposition spokesman Badr Jamous from the Syrian National Coalition claimed that some of the rockets delivered chemical payloads.
More than 1,300 people were killed, most of them by gas, said Khaled al-Saleh, another spokesman for the group.
Al-Saleh said that medical teams in the affected area had administered 25,000 shots of atropine -- a medication used to treat people exposed to the nerve gas sarin -- after the attack.
Doctors Without Borders said three hospitals it supports in Syria's Damascus governorate reported having received some 3,600 patients displaying neurotoxic symptoms last Wednesday morning.
"The reported symptoms of the patients, in addition to the epidemiological pattern of the events -- characterized by the massive influx of patients in a short period of time, the origin of the patients, and the contamination of medical and first aid workers -- strongly indicate mass exposure to a neurotoxic agent," said Dr. Bart Janssens, director of operations.
Video showed rows of bodies without apparent injury, as well as people suffering convulsions or who appeared to be struggling to breathe.
CNN could not verify where or when the videos were recorded, and could not confirm the number of casualties.
Opinion: Syria's lost youth -- one million child refugees flee
CNN's Jim Acosta, Ben Brumfield, Elise Labott, and Jill Dougherty contributed to this report.
