Airstrikes had targeted Saraqeb, a rebel-held town in the northwest of the country close to the front line with government forces, the Syrian Observatory for Human Rights said on Sunday.

The Syrian American Medical Society reported that its hospitals in the area were treating 11 other patients for chlorine gas poisoning.

Volunteer search and rescue workers with the "White Helmets" posted a video on Sunday which they said showed their teams responding after an "attack with chlorine gas."

The Syrian Civil Defense search-and-rescue group said on Sunday that three of its rescuers and six others had been injured in Saraqeb.

The UK-based Observatory reported a "foul smell after regime helicopters struck several areas of the town in Idlib province, causing five civilians to suffer from suffocation." It quoted residents and medical sources saying "toxic gas" had been used in the attack.

US accuses Damascus of foul play

The Syrian foreign ministry denied accusations made by the US last week that the Syrian regime had used chemical weapons on opposition forces near the capital Damascus in January.

US Defense Secretary Jim Mattis told said on Friday the US was concerned sarin gas may have been used, citing reports from NGOs and rebel groups.

These reports said toxic gas has been used, although Mattis said the US had no proof to support these accusations.

The Trump administration last week accused Syrian President Bashar Assad's government of producing and using "new kinds of weapons" to deliver deadly chemical poisons. The US has also suggested Moscow is involved.

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

Airstrikes continue

Syrian regime airstrikes killed six civilians in the town of Maarat al-Numan in the southern part of Idlib province, the Observatory said, damaging the main hospital.

The facility had stopped working until repairs could be carried out, it said.

jbh/jm (AFP, AP)