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

Rebel fighters take positions at the frontline during what they said were clashes with Islamic State militants in the town of Marea in Aleppo's countryside October 3, 2014. REUTERS/Rami Zayat

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 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 sulfur 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 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. 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.

ASSAD’S FORCES ALSO UNDER SUSPICION

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,” U.S. State Department spokesman John Kirby said, referring to President Bashar al-Assad’s government.

The White House said on Friday it was very concerned about the initial findings of the OPCW.

“We are continuing to investigate these allegations very closely and to be proactive about the threat from chemical weapons, or other similar threats,” a National Security Council spokesperson said in an email.

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 Nov. 23, sources at the OPCW told Reuters.

Sulfur 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.

The mission “cannot confidently determine whether or not a chemical was used as a weapon” by militants in the Jober area on Aug. 29, 2014, it said.

Syria agreed in September 2013 to destroy its entire chemical weapons program under a deal negotiated with the United States 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.

With Syria’s civil war in its fifth year, chlorine has also been used illegally in systematic attacks against civilians, the OPCW found.

A U.N.-OPCW joint investigative mission has been assigned to determine who was behind those attacks.

The three reports will be formally presented to U.N. Secretary General Ban Ki-moon later this month.