According to an foreign watchdog and other sources, mustard gas was used during clashes between ISIS militants and another rebel group in Syria, Reuters reported Thursday. A confidential Organization for the Prohibition of Chemical Weapons report found that “with the utmost confidence that at least two people were exposed to sulfur mustard” in the Syrian town of Marea on Aug. 21. “It is very likely that the effects of sulfur mustard resulted in the death of a baby,” the report said. The revelation confirms the use of the poisonous gas in Syria for the first time since the country agreed to destroy its chemical-weapons stockpile 18 months ago. The report did not mention ISIS, but diplomatic sources told Reuters the toxin had been used in fighting between rebel groups at that time. It’s unclear if ISIS made the chemical weapon or seized control of a stockpile.