A report by an international chemical weapons watchdog group says mustard gas was used during summer fighting in a Syrian town where Islamic State (IS) insurgents were battling a rebel group.

The deadly gas was used in the town of Marea in the northern province of Aleppo on August 21, a source from the Organization for the Prohibition of Chemical Weapons (OPCW) told Reuters.

“We have determined the facts, but we have not determined who was responsible,” the source said.

On August 25, Doctors Without Borders (also known in French as Médecins Sans Frontières, or MSF) released a statement saying it treated four members of the same family who showed "symptoms of exposure to chemical agents" on August 21.

The MSF statement said the patients were from the town of Marea and came to a hospital run by MSF in the Syrian province of Aleppo.

Syria's government agreed to surrender its entire chemical-weapons supply under an agreement with the United States and Russia in September 2013. This was after the US threatened military action following a deadly sarin gas attack on a rebel-held suburb of Damascus that killed hundreds of people.

The last of Syria's acknowledged 1,300-ton chemical weapons stockpile, which included mustard gas and raw materials for making sarin nerve gas, was handed over in June 2014 to OPCW for destruction. But several Western governments have expressed doubt that the government of President Bashar al-Assad declared and handed over its complete stockpile.

Ahmet Uzumcu, the director general of the OPCW, said that it is possible Syria has avoided declaring some part of its arsenal, The Associated Press reported.

"I can't say that Syria doesn't have any chemical weapons anymore," he said. However, he added that is true for any country that his organization cooperates with, and Syria's declared arsenal was close to estimates made by external security analysts and experts.

The latest OPCW report is the first confirmation of mustard gas being used in Syria since the country agreed to destroy its chemical arsenal.

"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."

Sulphur mustard, commonly known as “mustard gas”, can cause serious injuries and is listed as a Schedule 1 chemical agent, meaning it has few uses outside warfare.

In the recent past, there have been growing fears that IS is using chemical weapons in both Syria and Iraq.

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In August, the Pentagon said the militant group was suspected of having used them in an attack on Kurdish fighters in northern Syria.

Though the US has been reluctant to wage on ground direct combat with ISIS, President Obama has in the past said that use of chemical weapons would cross a red line that could force American action, the New York Times reports.