At least 70 people have died in a suspected chemical attack in Douma, the last rebel-held town in Syria’s Eastern Ghouta.
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.
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 Organisations, a US-based charity that works with Syrian hospitals, told the BBC the Damascus Rural Specialty Hospital had confirmed 70 deaths.
A spokeswoman said reports on the ground suggested a much higher number of around 180 dead, but that it was hard to reach victims.
She said there were reports of people being treated for symptoms including convulsions and foaming of the mouth, consistent with nerve or mixed nerve and chlorine gas exposure.
Continued shelling overnight and on Sunday (today) was making it impossible to reach victims.
As the allegations emerged, Syria’s state news agency Sana said the reports were invented by the Jaish al-Islam rebels who remain in control in Douma.
In August 2013, rockets containing the nerve agent Sarin were fired at rebel-held areas of the Eastern Ghouta, killing hundreds of people.
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 Organisation for the Prohibition of Chemical Weapons (OPCW) held the Syrian government responsible.
Activists, medics and the US said 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 previously found that government forces have used chlorine as a weapon at least three times during the seven-year civil war.
Douma is the last rebel-held town in Syria’s Eastern Ghouta region, and is under siege from Russian-backed Syrian government forces.