Militants from Islamic State of Iraq and the Levant (Isil) have been accused of using mustard gas in the battle around Syria’s shattered second city of Aleppo.

Doctors working in Marea, 25km north of the city, said they had treated more than 30 patients with suppurating blisters in the wake of Isil shelling.

The jihadist militants have launched a series of small-scale chemical attacks on Kurdish forces in Iraq in recent weeks, raising fears that chemical stockpiles belonging to deposed dictator Saddam Hussein and Syrian president Bashar al-Assad may have been pillaged by Isil.

The attack follows a similar one the week before last, in which Kurdish troops in northern Iraq complained of symptoms similar to those used in mustard gas attacks.

 “The fact that they (artillery shells) are fairly undamaged clearly shows they didn't contain a large explosive payload, which suggests another payload, possibly a chemical agent,” said Elliot Higgins, a British blogger specialising in open source weapons identification.

 A doctor working at a Syrian-American Medical Society clinic in Aleppo said that 30 of his patients had experienced blistering, as well as swollen eyes and breathing difficulties. Photographs released later showed bulbous blisters pushing out from a victim's back.