The Britain-based Syrian Observatory for Human Rights 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 as saying "toxic gas" was used in the attack, without elaborating.

Syrian regime air strikes also killed six civilians in the town of Maarat al-Numan in the southern countryside of Idlib province, the Observatory said, causing damage to the main hospital there.

The facility had stopped working 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.

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.

Matis 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.