Chemical weapons experts have determined mustard gas was used in a Syrian town where Islamic State (IS) insurgents were battling another group.

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

"It is very likely 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 IS, as the fact-finding mission was not mandated to assign blame, but diplomatic sources said the chemical had been used in the clashes between IS 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 (IS) 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.

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

Kurdish authorities said earlier this month IS 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.

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.

In the Idlib Province south of Aleppo, another report said, there were several incidents between March and May of 2015 which "likely involved the use of one or more toxic chemicals," including chlorine.

Those attacks, which resulted in the deaths of six people in the opposition-controlled region, have been blamed on government forces.

"Witnesses reported hearing helicopters overhead at the time the chemical munitions exploded. Only the Assad regime has helicopters," State Department spokesman John Kirby said, referring to President Bashar al-Assad's government.

A special session has been called by the OPCW's 41-member Executive Council to discuss the Syrian findings and it will be held in The Hague on November 23 when the reports will be handed to UN Secretary General Ban Ki-moon.

Sulphur mustard - which causes severe delayed burns to the eyes, skin and lungs - is a so-called Schedule 1 chemical agent, meaning it has few uses outside warfare.

The third report by the OPCW fact-finding mission to Syria said the team had so far been unable to substantiate claims from the Syrian government that its forces had been targeted by insurgents using chemical weapons.

Syria agreed in September 2013 to destroy its entire chemical weapons programme under a deal negotiated with the US and Russia after hundreds of people were killed in a sarin gas attack in the outskirts of the capital, Damascus.

The last of 1,300 tonnes of chemical weapons declared to the OPCW was handed over in June, 2014, but several Western governments have expressed doubt that the government of President Bashar al-Assad declared its entire arsenal.