(FOX NEWS) -- A poison gas attack late Saturday on a rebel-held town near Syria's capital has killed at least 40 people, activists, rescuers and medics said.

The alleged attack in Douma, about 10 miles east of Damascus, occurred amid a resumed offensive by Syrian government forces after the collapse of a truce.

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

In some cases, entire families were found suffocated in their houses and shelters, according to opposition-linked first responders known as the White Helmets.

The Syrian American Medical Society (SAMS) said 41 people had been killed, but other reports put the death toll much higher, Reuters reported.

Meanwhile, the Russian-backed Syrian government denied that its forces had launched any chemical attack, Reuters reported. The government said rebels in Douma, in eastern Ghouta, were in a state of collapse and spreading false news, the report said.

None of the reports could be immediately independently confirmed, both the Associated Press and Reuters said.

The U.S. State Department said it was closely following "disturbing reports regarding alleged chemical weapons attack targeting a hospital in Douma, Syria.

"The Assad regime and its backers must be held accountable and any further attacks prevented immediately," State Dept. spokeswoman Heather Nauert tweeted.

Activists circulated a video showing presumably dead men, women and children, some with foam at the mouth, the news agency added.

“Douma city, April 7 ... there is a strong smell here,” a voice is heard saying, the report said.

Just days ago, President Donald Trump said he was hoping to bring U.S. troops out of Syria within six months.

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