BEIRUT - An alleged gas attack killed at least 80 people in the eastern suburbs of Damascus, local responders said on Sunday. Syrian state media, meanwhile, reported that rebels there have agreed to give up their last foothold in the area.

First responders and a relief organisation said they found families suffocated in their homes and shelters in the besieged town of Douma, with foam on their mouths. They did not identify the substance used, but the Syrian Civil Defence and the Syrian American Medical Society said survivors treated at clinics smelled strongly of chlorine.

The reports, which started circulating late Saturday, could not be independently verified, and the government denied allegations it had used chemical weapon in its assault on the town.

Meanwhile, state news agency SANA said the Army of Islam group agreed to leave Douma on Sunday, after three days of intensive government shelling and bombardment. SANA said buses had been sent to the town to pick up prisoners released by the militants and to transport rebel fighters to opposition-held territory in north Syria. The Army of Islam could not be immediately reached for comment.

Talks to surrender Douma collapsed on Friday, leading to the government to restart its campaign to take the town after ten days of calm.

The Syrian Civil Defence first responder group documented 42 fatalities but was impeded from searching further by strong odors that gave their rescuers difficulties breathing, said Siraj Mahmoud, a spokesman for the group, which is known as the White Helmets.

Douma has been devastated by close to five years of siege at the hands of government forces. It was once one of the hubs of the 2011 Arab Spring-styled uprising against President Bashar Assad's government.

In recent weeks, government forces have recaptured villages and towns in the eastern Ghouta suburbs of the capital. Douma is the only town left holding out.

A joint statement by the Civil Defence and the Syrian American Medical Society, a relief organisation, said that more than 500 people, mostly women and children, were brought to medical centres with difficulty breathing, foaming at the mouth, and burning sensations in the eyes. It said patients gave off a chlorine-like smell. Some had bluish skin, a sign of oxygen deprivation.

It said the symptoms were consistent with chemical exposure. One patient, a woman, had convulsions and pinpoint pupils, suggesting exposure to a nerve agent.

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 them.

"Until this minute, no one has been able to find out the kind of agent that was used," said Mahmoud, the White Helmets' spokesman, in a video statement from Syria.

He said the government was also targeting homes, clinics, and first responder facilities with conventional explosives and barrel bombs. Most of the medical points and ambulances of the town have been put out of service.

Videos posted online by the White Helmets 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, calling it a "failed attempt" to impede government advances.

US President Donald Trump on Sunday said there will be a "big price to pay" after what he called a "mindless CHEMICAL attack" in Syria, allegedly involving chlorine gas. Trump also called Syrian President Bashar al-Assad an "animal."

"President Putin, Russia and Iran are responsible for backing Animal Assad. Big price to pay," Trump said in a pair of tweets which began with a discussion of the attack in Syria's Eastern Ghouta.

"Many dead, including women and children, in mindless CHEMICAL attack in Syria. Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world," the president said.

"Open area immediately for medical help and verification," Trump said. "Another humanitarian disaster for no reason whatsoever. SICK!"

The latest alleged attack came a year after the rebel-held town of Khan Sheikun in northwestern Syria was hit by an air strike. A UN-commissioned report said many residents of the town suffered the symptoms of an attack from an illegal nerve agent and more than 80 of them died, convulsed in agony.

Trump responded to that attack three days later, when US warships in the Mediterranean fired 59 cruise missiles at a Syrian airbase.

Trump on Sunday criticised his predecessor Barack Obama for not striking after warning that the use of chemical weapons in Syria was a "red line."

"If President Obama had crossed his stated Red Line in The Sand, the Syrian disaster would have ended long ago! Animal Assad would have been history!" Trump said.

Moscow also rejected claims the Syrian regime used chemical weapons on an opposition holdout in Eastern Ghouta, after the US said Russia bore ultimate responsibility for any attack.

"We firmly deny this information," said Major General Yuri Yevtushenko, head of the Russian Center for Reconciliation of the Warring Sides in Syria, in comments reported by news agencies.

"We are ready, once Douma is freed from militants, to immediately send Russian specialists in radiation, chemical and biological defence to collect data that will confirm these claims are fabricated," he added.