However, the government said: “If action in the Security Council is blocked, the UK would still be permitted under international law to take exceptional measures” as “humanitarian intervention.”
French President Francois Hollande, whose government was the first to speak of the possible use of force against Syria over the gas attacks, said the world must act to stop the violence.
However, a government spokeswoman said coordinating an agreed response was “difficult.”
Syrian opposition leader Ahmad al-Jarba, on a visit to Paris, said the West must get rid of al-Assad and his “killing machine” and bring him to trial at the International Criminal Court.
The international community has remained largely impotent over the war despite the huge death toll, as Russia, along with China, have blocked three Security Council resolutions aimed at pressuring al-Assad.
However, fears Western intervention could ignite a regional conflagration were stoked further by Iran, whose army chief Hassan Firouzabadi warned that a strike on Syria would “drive the Zionists to the edge of fire” and cause untold losses among the US and British ranks.
Israeli President Shimon Peres vowed that the Jewish state would “respond with all our might” if attacked.