Rescuers and medics in Syria say a poison gas attack on a rebel-held town near the capital has killed at least 40 people.

The alleged attack in the town of Douma occurred Saturday night amid a resumed offensive by Syrian government forces after the collapse of a truce.

The Syrian Observatory for Human Rights said at least 80 people were killed in Douma Saturday, including around 40 who died from suffocation.

Opposition-linked first responders, known as the White Helmets, also reported the attack, saying entire families were found suffocated in their houses and shelters.

It reported a death toll from suffocation of more than 40.

The Syrian American Medical Society, a relief organisation, says 41 people were killed and hundreds wounded.

Government forces resumed their offensive on rebel-held Douma on Friday afternoon after a 10-day truce collapsed over disagreement regarding the evacuation of opposition fighters.

Violence returned days after hundreds of opposition fighters and their relatives left Douma towards rebel-held areas in northern Syria. Douma is the last rebel stronghold in eastern Ghouta.

The alleged gas attack in Douma comes almost exactly a year after a chemical attack in the northern Syrian town of Khan Sheikhoun killed dozens of people.

That attack prompted US President Donald Trump to order a missile attack on a Syrian air base. The Syrian government and its ally, Russia, denied any involvement in the alleged gas attack.