Six people, including three young children, were killed in an alleged regime gas attack in northwestern Syria late Monday, a monitoring group and opposition activists said.
"Three children, their mother and father, and their grandmother suffocated to death after regime barrel bomb attacks" on their village in the province of Idlib, the Syrian Observatory for Human Rights said.
The Britain-based monitoring group said doctors in the village of Sarmin, southeast of the city of Idlib, concluded that the manner of death indicated a gas, possibly chlorine, had been emitted from the barrel bombs.
Sarmin's local coordinating committee, an activist group, said chlorine gas had been used and published videos of a chaotic field hospital where disoriented victims coughed and held gas masks over their faces as children cried in the background.
It also published videos of people trying to revive three young children who lay motionless in the field hospital. They appeared pale and covered in sweat, with dark circles around their eyes and a clear liquid coming out of their noses and mouths.
The three children, who all appeared to be no older than three, were pronounced dead. Pictures of their tiny bodies wrapped in white cloth were shared widely on social media.
Activists have accused the Syrian regime of using chlorine -- a toxic agent that can be considered a chemical weapon -- on civilian areas in the past.
In January, the Organisation for the Prohibition of Chemical Weapons said chlorine gas had been used in attacks on three Syrian villages in 2014.
The watchdog said it had a "high degree of confidence" that the gas had been used and reported that at least 13 people had been killed in the attacks.
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It did not however attribute responsibility for the attacks, though its report cited witnesses saying they heard helicopters, which only the regime possesses, before the bomb attacks occurred.
Syria's opposition National Coalition said the regime's use of chlorine violated a UN Security Council resolution adopted in March after the OPCW's report.
The resolution condemned use of chlorine in the conflict and threatened action if it was used again.
Opposition chief Khaled Khoja condemned the attack on Twitter, writing: "The psychopathic dictator (Bashar al-Assad) continues to kill by chemicals and challenges the international laws".
Assad's regime has been criticised by human rights groups for using barrel bombs, crudely constructed weapons packed with explosives typically dropped by helicopter.
Ibrahim al-Idlibi, an activist from the area, confirmed that six civilians had died of suffocation after two rounds of barrel bombs were dropped on the village.
"In the second round, civil defence volunteers were trying to protect the civilians by splashing water on them. But this is when they started suffocating," he said.
Damascus did not have to declare its stocks of chlorine under a 2013 agreement to dismantle its chemical arsenal as it is widely used for commercial and domestic purposes.
But use of the gas for military purposes would be a breach of its undertakings under the Chemical Weapons Convention, which it signed as part of the deal.