Russia and China have again vetoed a Western-backed UN resolution threatening non-military sanctions against Syria.

It was aimed at stepping up pressure on President Bashar Assad's government to end the escalating 16-month conflict.

Today's 11-2 vote, with two abstentions, leaves in limbo the future of the 300-strong UN peacekeeping force in Syria, whose mandate expires tomorrow.

Britain's UN Ambassador Mark Lyall Grant, who sponsored the Western-backed draft, said he was "appalled" at the third double veto by Russia and China, allies of the Assad regime.

There were two abstentions from South Africa and Pakistan.

It was a blow to Kofi Annan, the joint UN-Arab League envoy to Syria, who had called for "consequences" for non-compliance with his six-point peace plan, which the Assad government has flouted.

The resolution called on Mr Assad to withdraw troops and heavy weapons from populated areas within 10 days.

Mr Annan had requested a delay in yesterday's vote on the resolution and appealed to the council to unite behind a new resolution, but Moscow would not budge and the West insisted on including the threat of non-military sanctions under Chapter 7 of the UN Charter.

That could eventually open the door to the use of military force.