Rescuers in Syria say at least 150 people have died after toxic gas was dropped on Douma, the last rebel-held town in Eastern Ghouta.
Volunteer rescue force The White Helmets tweeted graphic images showing scores of bodies in a basement. It said the death toll is likely to rise.
There has been no independent verification of the reports.
Syria’s government has called the allegations of a chemical attack a “fabrication”.
The US state department said it is monitoring the reports, and that Russia – which is fighting alongside the Syrian government – should be held responsible if deadly chemicals had been used.
“The regime’s history of using chemical weapons against its own people in not in dispute,” said the State Department.
“Russia ultimately bears responsibility for the brutal targeting of countless Syrians with chemical weapons.”
The pro-opposition Ghouta Media Center tweeted that more than a 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.
Douma is the last rebel-held town in Syria’s Eastern Ghouta region, and is under siege from government forces.