BEIRUT: Syrian opposition activists and rescuers said Sunday that a poison gas attack on a rebel-held town near the capital has killed at least 40 people, allegations denied by the Syrian government. The alleged attack in the town of Douma occurred late Saturday amid a resumed offensive by Syrian government forces after the collapse of a truce. The reports could not be independently verified. Opposition-linked first responders, known as the White Helmets, reported the attack, saying entire families were found suffocated in their homes and shelters. It reported a death toll from suffocation of more than 40, saying the victims showed signs of gas poisoning including pupil dilation and foaming at the mouth. In a statement, however, it reported a smell resembling chlorine, which would not explain the described symptoms, usually associated with sarin gas. It said around 500 people were treated for suffocation and other symptoms, adding that most medical facilities and ambulances were put out of service because of the shelling. The Syrian American Medical Society, a relief organization, said 41 people were killed and hundreds wounded. The Syrian Observatory for Human Rights said at least 80 people were killed in Douma on Saturday, including around 40 who died from suffocation. But it said the suffocations were the result of shelters collapsing on people inside. Videos posted online by the White Helmets purportedly showed victims, including toddlers in diapers, breathing through oxygen masks at makeshift hospitals. The Syrian government, in a statement posted on the state-run news agency SANA, strongly denied the allegations. It said the claims were "fabrications" by the Army of Islam rebel group, calling it a "failed attempt" to impede government advances. "The army, which is advancing rapidly and with determination, does not need to use any kind of chemical agents," the statement said. Syrian government forces resumed their offensive on rebel-held Douma on Friday afternoon after a 10-day truce collapsed over disagreement regarding the evacuation of Army of Islam fighters. Violence resumed days after hundreds of opposition fighters and their relatives left Douma toward 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 the US to launch several dozen Tomahawk cruise missiles at a Syrian air base. President Donald Trump said the attack was meant to deter further Syrian use of illegal weapons. The Syrian government and its ally, Russia, denied any involvement in the alleged gas attack. Douma is in the suburbs 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 ever 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.