The United States said it was "deeply troubled" Wednesday after the world's chemical watchdog confirmed the systematic use of chlorine as a weapon in war-torn Syria.
President Bashar al-Assad's regime and the rebels fighting it have both accused the other of using chemical agents, including chlorine, in the bloody uprising that began in March 2011.
Damascus promised to hand over all its chemical arms, and tonnes of chemical agents have been destroyed by international monitors.
But a mission by the Organization for the Prohibition of Chemical Weapons found "compelling confirmation" that a toxic chemical was used "systematically and repeatedly" as a weapon in villages in northern Syria earlier this year.
The OPCW said in a statement that the reporting team had been able "to conclude with a high degree of confidence that chlorine, either pure or in mixture, is the toxic chemical in question."
Chlorine is a widely available chemical that is non-persistent and so conclusively proving its use is a challenging task.
"We are deeply troubled by this report," said US State Department spokeswoman Marie Harf.
Syria did not have to declare its stockpile of chlorine -- a weak toxic agent that can be considered a chemical weapon if used offensively -- as part of a disarmament deal agreed last year because it is widely used for commercial and domestic purposes.
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All of Syria's declared stockpile of dangerous chemicals has either been destroyed in country or exported for destruction as part of a deal agreed a year ago in a bid to head off US-backed air strikes on the Syrian regime following a deadly chemical attack in Damascus.
Speaking in Washington, Harf added: "As we've said all along, we have concerns still along with the rest of the international community about the omissions related to Syria's chemical weapons declaration to the OPCW.
"Obviously, it's a good thing that a large mount of chemical weapons were removed, but this is an ongoing process and these concerns need to be addressed."
Harf said the OPCW report found that chemical attacks were from helicopters, prompting her to say: "At this point, the conclusion is that the Assad regime is responsible for the attacks, they are the ones with this helicopter capabilities.
"The fact that we were able to get a huge amount of chemical weapons out of Syria for destruction is an important milestone, but there is more work to do. And we do have serious concerns that remain."
The OPCW said that reports of chlorine attacks in Syria dropped off after the fact-finding mission was established in April, "but there was a spate of new allegations in August."
Spokesman Michael Luhan said the new claims would be investigated.
UN-mandated human rights investigators said in the last month they believed the Syrian government dropped chlorine on civilian areas on eight different occasions in April.