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

A confidential Oct. 29 report by the Organization 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.

The report did not mention ISIS, as the fact-finding mission was not mandated to assign blame, but diplomatic sources said the chemical had been used in the clashes between ISIS and another rebel group taking place in the town at the time.

“It raises the major question of where the sulfur mustard came from,” one source said. “Either they (ISIS) gained the ability to make it themselves, or it may have come from an undeclared stockpile overtaken by ISIS. Both are worrying options.”

Syria is supposed to have completely surrendered the toxic chemicals 18 months ago. Their use violates U.N. Security Council resolutions and the 1997 Chemical Weapons Convention.

The report, which will be formally presented to U.N. Secretary General Ban Ki-moon later this month, adds to a growing body of evidence that the ISIS has obtained, and is using, chemical weapons in both Iraq and Syria.

Kurdish authorities said earlier this month ISIS fighters fired mortar rounds containing mustard agent at Kurdish peshmerga fighters in northern Iraq during clashes in August. They said blood samples taken from around 35 fighters who were exposed in the attack southwest of the regional capital of Erbil showed “signatures” of mustard gas.