A chemical attack on a rebel-held town in eastern Ghouta killed dozens of people, a medical relief organisation and a rescue service said, and Washington said the reports - if confirmed - would demand an immediate international response.
A joint statement by the medical relief organisation Syrian American Medical Society (SAMS) and the civil defence, which operates in rebel-held areas, said 49 people had died.
The Russian-backed Syrian state denied government forces had launched any chemical attack as reports began circulating on Saturday night. The government said rebels in the eastern Ghouta town of Douma were collapsing and spreading false news.