Chemical weapons experts have determined that mustard gas was used in a Syrian town where Islamic State insurgents were battling another rebel group, according to a report by an international watchdog seen by Reuters.

A confidential Oct. 29 report by the Organisation for the Prohibition of Chemical Weapons (OPCW), a summary of which was shown to Reuters, concluded “with the utmost confidence that at least two people were exposed to sulfur mustard” in the town of Marea, north of Aleppo, on Aug. 21.

“It is very likely that the effects of sulfur mustard resulted in the death of a baby,” it said.

The findings provide the first official confirmation of use of sulfur mustard, commonly known as mustard gas, in Syria since it agreed to destroy its chemical weapons stockpile, which included sulfur mustard.