At least 70 people have died in a suspected chemical attack in Douma, the last rebel-held town in Syria's Eastern Ghouta, rescuers and medics say.
Volunteer rescue force the White Helmets tweeted graphic images showing several bodies in basements. It said the deaths were 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 reports suggested "a potentially high number of casualties", including families in shelters.
It said Russia - with its "unwavering support" for Syria's government - "ultimately bears responsibility" for the alleged attacks.
"The regime's history of using chemical weapons against its own people is not in dispute," it said in a statement.
Several medical, monitoring and activist groups reported details of a chemical attack, but figures vary and details of what happened are still emerging.
"Seventy people suffocated to death and hundreds are still suffocating," said Raed al-Saleh, head of the White Helmets. An earlier, now deleted tweet, put the number dead at more than 150.
The pro-opposition Ghouta Media Center tweeted that more than 75 people had "suffocated", while a further 1,000 people had suffered the effects of the alleged 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.