At least 70 people have died in a suspected chemical attack in Douma, the last rebel-held town in Syria's Eastern Ghouta, BBC reports citing to rescuers and medics.
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.