Syria's government has disposed of more than 86 percent of its total chemical weapons stockpile, the watchdog agency charged with overseeing its removal announced Tuesday.
But there are new reports alleging that Syria launched several chlorine gas attacks this month, potentially exposing a major loophole in the international deal to remove the chemical weapons.
President Bashar al-Assad agreed with the United States and Russia to dispose of his chemical weapons — an arsenal that Damascus had never previously formally acknowledged — after hundreds of people were killed in a sarin gas attack on the outskirts of the capital last August.
Washington and its Western allies said Assad's forces unleashed the nerve agent in the world's worst chemical attack in a quarter-century, while the Syrian government blamed the rebel side in Syria's three-year-old civil war.
Director-General Ahmet Uzumcu of the Organization for the Prohibition of Chemical Weapons (OPCW) said Syria has just surrendered another batch of raw materials used for making chemical weapons. Syria submitted a list of its chemical weapons to OPCW in the process of destroying them.
Syria has missed several deadlines for progress specified in last year's agreed timetable to eradicate its poison gas and nerve agent program by June 30. It insists it will meet the final deadline.
But chlorine gas was never included on the list submitted to the OPCW and is now allegedly being used on the battlefield, leading some countries to consider requesting an investigation, possibly through the United Nations.
Attacks this month in several areas of the country share characteristics that have led some analysts to believe that there is a coordinated chlorine campaign, with growing evidence that it is the government side dropping the bombs.
The U.S. State Department, which is examining the allegations, said on Tuesday that if the Syrian government used chlorine with the intent to kill or harm this would violate the Chemical Weapons Convention (CWC), which it joined as part of last September's Geneva agreement to give up its chemical weapons.
"The use of any toxic chemical with the intent to cause death or harm is a clear violation of the convention," said State Department spokeswoman Jen Psaki.
In the rebel-held village of Kfar Zeita in the central province of Hama, 125 miles north of Damascus, opposition activists uploaded video of people choking and being fed oxygen following what they said were bombs dropped from helicopters on April 11 and 12.
Reuters could not verify the authenticity of the videos, and activists regularly make similar claims, but further footage of canisters provided an indication of what had happened.
One of the canisters had only partially exploded, and the marking CL2 was written along its side. CL2 is the symbol for chlorine gas. Also visible was "Norinco" — China's biggest arms maker.
Repeated calls to China North Industries Group Corporation, or Norinco, went unanswered.
Canisters pictured in three separate areas were all painted yellow — complying with international standards on industrial gas color codes indicating chlorine.
Since April 11, there have been repeated attacks on Kfar Zeita and also on the town of Al-Tamana'a in northwest Idlib on Friday that shared the same characteristics.
Activists said helicopters dropped improvised barrel bombs with a chlorine canister enclosed, which led to casualties.
If inhaled, chlorine gas — a deadly agent widely used in World War One — turns to hydrochloric acid in the lungs, which can lead to internal burning and drowning through a reactionary release of water in the lungs.
Hamish de Bretton-Gordon, head of British-based chemical biological radiological and nuclear consultancy firm Secure Bio, said he was “reasonably satisfied that chlorine has been used” in Syria.
“The evidence is pretty compelling,” he said.
Al Jazeera and wire services