Updated 9.45am

A poison gas attack on a rebel-held town near the Syrian capital has killed at least 40 people, according to activists, rescuers and medics.

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

Photo: Twitter

The Syrian Observatory for Human Rights said at least 80 people were killed in Douma on 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, said 41 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.

Photo: Twitter

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.

Douma is in the suburb of Damascus known as eastern Ghouta. A chemical attack in eastern Ghouta in 2013 that was widely blamed on government forces killed hundreds of people, prompting the US to threaten military action before later backing down.

Syria denies using chemical weapons during the seven-year civil war, and says it eliminated its chemical arsenal under a 2013 agreement brokered by the US and Russia after the attack in eastern Ghouta.