Disturbing footage
Opposition activists said that more than 1,000 people were killed after government forces launched rockets with toxic agents into the Damascus suburbs in the Ghouta region early on Wednesday.
The BBC has been unable to independently confirm the death toll.
Activists said Wednesday's attack took place as part of heavy government bombardment in the region surrounding Damascus, with government forces trying to drive out rebel forces. The areas affected included Irbin, Duma and Muadhamiya.
Video footage shows dozens of bodies with no visible signs of injuries, including small children, and survivors being treated in makeshift hospitals, with victims, including many children, having convulsions.
Chemical weapons experts have told the BBC that footage appears genuine and that the injuries shown are consistent with nerve agents.