Chemical weapons attacks have killed dozens on the outskirts of Damascus, Syrian opposition activists claim.
Rockets with toxic agents were launched at the suburbs of the Ghouta region early on Wednesday as part of a major bombardment on rebel forces, they say.
A team of UN inspectors arrived in Syria on Sunday to probe earlier allegations of chemical weapons use.
Syria's state-run news agency said that the reports were "baseless" and an attempt to distract the inspectors.
UK Foreign Secretary William Hague expressed "deep concern" and called for the Syrian government "to allow immediate access to the area" for the investigators.