UN-Arab League envoy Kofi Annan asked the UN Security Council to delay a vote scheduled for Wednesday on a Western-drafted resolution calling for sanctions against Syria, British diplomats said.

"Joint Special Envoy Annan has asked to delay today's draft Syria resolution vote. With fellow co-sponsors we're considering that request," a British spokesman said in a statement posted on Twitter.

"Annan feels it is still possible to get a compromise with Russia on the resolution," said a diplomat from another UN Security Council nation, confirming the move.

Foreign ministers from the five permanent Security Council members -- Russia, United States, France, Britain and China -- were also negotiating the future of the resolution that Russia has vowed to veto, diplomats said.

UN ambassadors from the five countries planned to hold their own talks before announcing whether the vote would go ahead, diplomats said.

Britain, with the backing of France, United States, Germany and Portugal, has proposed a resolution under Chapter 7 of the UN Charter, which threatens non-military sanctions if President Bashar al-Assad does not halt the use of heavy weapons within 10 days of a resolution being passed.

Russia fiercely opposes the threat of sanctions.

More than 17,000 people have been killed since a popular uprising against Assad began 16 months ago, activists say.