The US says it fears Syria's President Bashar al-Assad may resort to using chemical weapons against his people. White House spokesman Jay Carney said the US was concerned that such an action might be taken by "an increasingly beleaguered regime".
A Syrian official has insisted it would "never, under any circumstances" use such weapons, "if such weapons exist".
Meanwhile, the United Nations says it was pulling "all non-essential international staff" out of Syria.