GENEVA — A toxic chemical, probably chlorine, was used as a weapon to attack Syrian villages in April, an international watchdog agency confirmed on Wednesday.

The conclusion, based on months of investigation by a fact-finding team, appeared to indicate that the Syrian government was continuing to use chemical weapons in the country’s civil war, despite having agreed to forswear the weapons, surrender its arsenal and tear down its manufacturing plants.

The agency, the Organization for the Prohibition of Chemical Weapons, said in a statement from its headquarters in The Hague that the information its team had collected provided “compelling evidence” that the toxic chemical was used “systematically and repeatedly” in Talmanes, Al Tamanah and Kafr Zet, three villages in northern Syria.

It said it had “a high degree of confidence that chlorine, pure or in mixture, is the toxic chemical in question,” based on the descriptions, physical properties, behavior of the gas, and signs and symptoms resulting from exposure, as well as the way victims responded to treatment.

The fact finders did not specify who had conducted the chlorine attacks. But its full report, which has been shown so far only to governments, is understood to leave little doubt that the Syrian government was responsible.

Michael Luhan, a spokesman for the organization, said in a telephone interview that witnesses cited in the report saw bombs dropped from high-flying helicopters that released the gas on impact. Of all the combatants in the civil war, only the Syrian government is known to have the ability to conduct such an aerial attack.

The fact-finding team said it was continuing to investigate reports of subsequent chlorine attacks, including a spate of new allegations in August.

Unlike nerve agents, mustard gas or other specially developed chemical weapons, chlorine is a common substance with many civilian and industrial uses. The international treaty banning chemical weapons does not restrict the manufacture or stockpiling of chlorine, but it does prohibit using it or any chemical as a weapon.

The organization’s report adds weight to the finding of a United Nations panel investigating human rights violations in Syria, which released a report last month saying that chlorine attacks had been carried out.

The fact-finding team went to Syria in May to investigate the reports of chlorine attacks, but soon withdrew, after a convoy of vehicles carrying its inspectors was attacked with a roadside bomb and automatic weapons.

The organization said the team based its findings on dozens of interviews with victims, doctors, emergency medical workers and witnesses to the attacks, as well as a considerable amount of video, medical records and other documentation.

Under intense international pressure and a threat of American airstrikes, Syria agreed last year to give up its chemical weapons for destruction. The deal, brokered by the United States and Russia, followed reports of a deadly sarin gas attack on a Damascus suburb in August 2013 that Washington believes killed some 1,400 people.

Destruction of Syria’s declared stockpile of toxic agents and precursor chemicals was completed last month. But Western governments continue to express concerns that Syria did not declare its whole stockpile and may still have some deadly chemicals that could fall into the hands of Islamic extremists. Read more….