Activists and medical organisations have documented an alleged chemical weapons attack on a Syrian town last week that affected dozens of civilians, with one source blaming the Islamic State group.
Doctors Without Borders (MSF) said it had treated civilians apparently exposed to a chemical agent in Marea, without saying what type or providing overall casualty figures.
The Syrian American Medical Society (SAMS) said its own doctors in the northern town had identified the agent as mustard gas.
Mamoun al-Khatib, a journalist and activist from Marea, said more than 50 artillery shells crashed into the centre of the town, an opposition stronghold in the northern province of Aleppo, before midday last Friday.
Khatib accused the Islamic State (IS) group, which has attempted to seize Marea in recent months, of firing the shells which he said emitted a "hideous stench".
"We went to the local field hospital, and the doctors told us there were cases of suffocation, severe coughing, redness in the eyes and face, and skin irritations," said Khatib, who heads Shahba, an anti-regime news agency.
"In total, more than 25 people were affected, four of them serious cases that were transferred to Turkey. They were all civilians," he said.
SAMS, which supports clinics in war-torn Syria, said a field hospital it operates in Marea treated "more than 50 civilians exhibiting symptoms of chemical exposure".
"Roughly 30 civilians developed skin blisters, with doctors identifying the agent to be mustard gas," it said in a statement, adding blood, clothing and hair samples had been collected from patients for further assessment.
