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.