A confidential October 29 report by the Organisation for the Prohibition of Chemical Weapons (OPCW), as per the reports, concluded "with the utmost confidence that at least two people were exposed to sulphur mustard" in the town of Marea, north of Aleppo, on August 21.

"It is very likely that the effects of sulphur mustard resulted in the death of a baby," it said. The findings provide the first official confirmation of use of sulphur mustard, commonly known as mustard gas, in Syria since it agreed to destroy its chemical weapons stockpile, which included sulphur mustard.

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

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 findings were part of three reports released to members of the OPCW last week. They add to a growing body of evidence that the Islamic State group has obtained, and is using, chemical weapons in both Iraq and Syria.

Kurdish authorities said earlier this month that Islamic State fighters fired mortar rounds containing mustard agent at Kurdish Peshmerga fighters in northern Iraq during clashes in August.

A team of OPCW experts has been sent to Iraq to confirm the findings and is expected to obtain its own samples later this month, one diplomat said.

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