Russia and China again vetoed a Western-backed U.N. resolution Thursday aimed at pressuring President Bashar Assad's government to end the escalating 16-month conflict in Syria.

The 11-2 vote, with two abstentions from South Africa and Pakistan, was the third double veto of a resolution addressing the Syria crisis by Damascus' most important allies.

The defeat leaves in limbo the future of the 300-strong U.N. observer mission in Syria, which was forced to suspend operations because of the intensified fighting. Its mandate, to monitor a cease-fire and implementation of international envoy Kofi Annan's peace plan, expires Friday.

Britain's U.N. Ambassador Mark Lyall Grant, who sponsored the Western-backed draft, said he was "appalled" at the third double veto of a resolution aimed at bringing an end to the bloodshed in Syria and creating conditions for political talks. The resolution had threatened sanctions if the Syrian regime didn't quickly stop using heavy weapons.