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 has called the allegations of a chemical attack a "fabrication" - as has its main ally, Russia.
The US state department said Russia - with its "unwavering support" for Syria's government - "ultimately bears responsibility" for the alleged attacks.
