Activists and a medic in war-torn Syria on Friday claimed President Bashar al-Assad's regime used chlorine gas a day earlier to attack opposition-held towns in Hama and Idlib provinces.

The reports could not be confirmed by independent sources, including the Syrian Observatory for Human Rights, a Britain-based monitoring group.

"Kafr Zita in Hama province was hit on Thursday with barrel bombs (dropped from helicopters) that were laden with chlorine gas," according to an activist in the Syrian Revolutionaries Union reached by AFP.

"The attack was because the regime wanted to take revenge against the town for allowing the rebels to use it as a base for operations against the army in Hama countryside," Mohammad Karman said.

Activists in Idlib province, in northwestern Syria, meanwhile distributed videos showing patients, including children, being treated in field hospitals for apparent suffocation.

Doctor Tajeddin al-Bakri, a doctor from the area, told AFP: "We are sure chlorine was used. You can smell chlorine from miles away, and the gas emanating from the explosion (after the barrel bomb attack) was a white, yellowish colour."

Western backers of Syria's anti-Assad revolt and rights groups have previously said the regime is believed to have used chlorine gas to attack rebel-held areas.

The United States has said that, should such allegations be proven, the attacks would have been in contravention of the Chemical Weapons Convention that Damascus signed up to as part of a US-Russian deal to clear Syria of its chemical arsenal.

The Organisation for the Prohibition of Chemical Weapons watchdog said last month it would look into the attacks and has dispatched a team to carry out investigations on the ground in Syria.