Chlorine gas left civilians gasping for air in a rebel-held Syrian town after a chemical attack as Bashar al-Assad's regime launched a wave of airstrikes, activists have claimed.

Victims had to be hosed down with water after being poisoned in the helicopter attack on Saraqeb in Idlib province.

The Britain-based Syrian Observatory for Human Rights reported a 'foul smell after regime aircraft struck several areas of the town, causing civilians to suffer from suffocation'.

Chlorine gas left civilians gasping for air in a rebel-held Syrian town after a chemical attack as Bashar al-Assad's regime launched a wave of airstrikes, activists have claimed. Pictured: rescuers at work after a strike on Kafr Nabl in Idlib

It quoted residents and medical sources as saying 'toxic gas' was used in the attack, without elaborating.

Syrian regime air strikes also killed six civilians in the town of Kafr Nabi, near Maaret al Numan in the southern countryside of Idlib province, the Observatory said.

Four other civilians were killed in regime bombing on Maaret al Numan and Maasarin.

The Observatory also said that 'airplanes believed to be Russian hit the main hospital in Maaret al Numan causing damage'.

The facility shut down until repairs could be carried out, the war monitor said.

The latest developments come as the United States this week accused the Syrian regime of using chemical weapons on opposition forces near the capital Damascus.

Syrian regime air strikes also killed six civilians in the town of Kafr Nabi (pictured), near Maaret al Numan in the southern countryside of Idlib province, the Observatory said

The Syrian foreign ministry denied the accusations as 'lies'.

US Defence Secretary Jim Mattis told reporters on Friday his government was concerned sarin gas may have been recently used in Syria, citing reports from NGOs and rebel groups.

These reports said toxic gas has been used.

Mattis however said the United States has no proof to support these accusations.

Last month, 21 people were treated for respiratory problems after rockets were fired on the rebel bastion of Eastern Ghouta outside Damascus.