Rescuers and medics say at least 70 people have died in Syria in a suspected gas attack in Douma, the last rebel-held town in Eastern Ghouta.
Volunteer rescue force the White Helmets on Saturday tweeted graphic images showing several bodies in a basement. It said the death toll is likely to rise.
There has been no independent verification of the reports, according to a BBC report on Sunday.
Syria's government has called the allegations of a chemical attack a "fabrication".
The US state department said it is monitoring the "very disturbing" reports, and that Russia - which is fighting alongside the Syrian government - should be held responsible if deadly chemicals had been used.
Several medical, monitoring and activist groups reported details of a chemical attack, but figures vary and what happened is still becoming clear.
The pro-opposition Ghouta Media Center tweeted that more that 75 people had "suffocated" while a further thousand people had suffered the effects of the alleged gas attack.
It blamed a barrel bomb allegedly dropped by a helicopter which it said contained sarin, a toxic nerve agent.
The Union of Medical Relief Organizations, a US-based charity that works with Syrian hospitals, told the BBC the Damascus Rural Specialty Hospital had confirmed 70 deaths.
Has the Syrian government used gas before?
In August 2013, rockets containing the nerve agent sarin were fired at rebel-held areas of Eastern Ghouta, killing hundreds of people.
A UN mission confirmed the use of sarin, but it was not asked to state who was responsible. Western powers said only Syrian government forces could have carried out the attack.
In April 2017, more than 80 people died in a sarin attack on the opposition-held town of Khan Sheikhoun, and a joint inquiry by the UN and the Organization for the Prohibition of Chemical Weapons (OPCW) held the Syrian government responsible.
Activists, medics and the US say Syrian government forces dropped bombs containing toxic chlorine gas on rebel-held towns in early 2018.
The joint UN-OPCW mission is investigating the reports. It has previously found that government forces have used chlorine as a weapon at least three times during the seven-year civil war.