THE HAGUE- 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 sulphur mustard" in the town of Marea, north of Aleppo, on Aug. 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."It raises the major question of where the sulphur 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 IS. Both are worrying options."Syria is supposed to have completely surrendered the toxic chemicals 18 months ago. Their use violates UN Security Council resolutions and the 1997 Chemical Weapons Convention.In a statement to Reuters, State Department spokesman John Kirby said the OPCW released three reports to member states on Oct. 29, adding "two of these reports are very disturbing. The OPCW has confirmed (chemical weapons) use in one investigation, and concluded likely use in a second."