Chemical weapons experts confirmed Thursday that a chemical agent known as mustard gas was used in a Syrian town where "Islamic State" insurgents were battling another rebel group, according to a leaked report by the Organization for the Prohibition of Chemical Weapons (OPCW).

"We have determined the facts, but we have not determined who was responsible," an unnamed OPCW official said.

A confidential October 29 report by the OPCW, a summary of which was shown to news agencies, 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.

Doctors without Borders said it had treated four members of a single family. Patients at a the medical charity's hospital in Aleppo said a mortar round hit their house and "after the explosion a yellow gas filled the living room."

In a US-Russia-brokered bid to prevent US military intervention, Syria agreed in September 2013 to destroy its entire chemical weapons program following a deadly sarin gas attack killed hundreds of people in the outskirts of Damascus.

United Nations awaits report

The last of 1,300 tons 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 complete stockpile.

The report, which will be formally presented to UN Secretary-General Ban Ki-moon later this month, adds to a growing body of evidence that the Islamic State militant group is waging war with chemical weapons in both Iraq and Syria.

Kurdish authorities said earlier this month that IS had fired mortar rounds containing mustard gas 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 traces of the chemical agent.

A special session has been called by the 41-nation OPCW Executive Council to discuss the Syrian findings. A meeting is scheduled for November 23 in The Hague.

jar/sms (AFP, Reuters)