At least 19 people were killed in several villages and towns in northwestern Syria after government forces launched barrel and gas attacks, activists and monitoring groups said.

Three children, their mother and father, and their grandmother suffocated to death after the barrel bomb attack in the town of Sarmin in southeast Idlib province, the Syrian Observatory for Human Rights said on Tuesday.

The Britain-based monitoring group told AFP news agency that doctors in Sarmin 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 photos of a chaotic field hospital where disoriented victims coughed and held gas masks over their faces.

It also posted videos of the pale bodies of the three children apparently killed in the attack. They appeared to have been no older than 5 and had dark circles around their eyes.

The Syrian government has been accused of using chlorine - a toxic agent that can be considered a chemical weapon - on civilian areas in the past.

It has also been criticised by rights groups for using barrel bombs, crudely constructed weapons packed with explosives and 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.

He told AFP that local doctors had not yet specified what the gas was.

In early March, the UN Security Council adopted a US-drafted resolution condemning the use of chlorine in Syria and threatening measures if chemicals are used in future attacks.

'Indication of war crimes'

Meanwhile, Al Jazeera has learned that 13 other people were killed and dozens were injured when helicopters dropped barrel bombs in the city of Khan Sheikhoun.

A child was also killed, and several others were injured in the town of Ma’arret Hormi because of similar attacks, which also targeted areas around the airbase of Abo al- Dohur, the town of Kafar Takharim and the city of Bennesh.

Activists on the ground reported that rescue teams in the region declared a state of full alert, and medical facilities have been overwhelmed with large numbers of wounded, including one of the rescue crews.

The attacks have reportedly forced many people to flee on foot to the nearby villages.

News of the latest attacks came as Amnesty International criticised government bombings last November that targeted the de-facto capital of the Islamic State of Iraq and the Levant (ISIL) in Raqqa.

Amnesty said in a statement that it has documented a series of Syrian government airstrikes between November 11 and November 29 that killed up to 115 civilians, including 14 children.

On November 25, The Associated Press reported that at least 60 people were killed in air strikes on that day alone.

Raqqa has been the seat of ISIL since the armed group declared a caliphate in areas under its control in Iraq and Syria.

"Syrian government forces have shown flagrant disregard for the rules of war in these ruthless air strikes. Some of these attacks give every indication of being war crimes,'' said Philip Luther, Director of the Middle East and North Africa Program.

Citing Syrian authorities, who at the time said the attacks were meant to target ISIL members and bases, Amnesty said the evidence it gathered shows that, in most cases, no military targets could be identified in the vicinity of the areas attacked.