The UN Security Council has scheduled a vote on a new Syria resolution after a last-minute delay failed to get key countries to agree on measures to end the dramatically escalating violence.
Britain's UN Ambassador Mark Lyall Grant said his country's text would be put to a vote later on Thursday morning.
It threatens non-military sanctions against President Bashar Assad's government if he doesn't withdraw troops and heavy weapons from populated areas within 10 days and is tied to Chapter VII of the UN Charter, which could eventually allow the use of force to end the conflict.
Russia, which is a close Syrian ally, says it will veto any Chapter VII resolution.
In Moscow on Wednesday, Russia's Foreign Minister Sergey Lavrov pointed to Wednesday's deadly bombing in the heart of Damascus that killed the defence minister and his deputy, Assad's powerful brother-in-law, and accused the West of inciting the Syrian opposition.
Russia is vehemently opposed to sanctions and any mention of Chapter VII and Lavrov argued that the British text amounted to support for the rebels and would lead to more bloodshed.
International envoy Kofi Annan contacted several governments on Tuesday and urged the council to postpone Wednesday's scheduled vote so members could "unite and take concerted and strong action that would help stem the bloodshed in Syria and build momentum for a political transition," his spokesman Ahmad Fawzi said.
Annan said Wednesday's bombing "only underscores the urgency of decisive council action," Fawzi said.
UN Secretary-General Ban Ki-moon, who was visiting China, also urged the Security Council to "take collective action, with a sense of unity".
After a phone call from President Barack Obama to Russian President Vladimir Putin on Wednesday afternoon, the Russian and US ambassadors met at the United Nations but there was no breakthrough.
Nonetheless, diplomats said there was still a last-minute chance for compromise.
"Who knows where we're going to end up," US Ambassador Susan Rice told reporters.
If the Western-backed draft resolution is vetoed, Russia could then put its rival text - which makes no mention of sanctions or Chapter VII - to a vote.
But diplomats said Moscow doesn't have the minimum nine "yes" votes required for Security Council approval, so that appears unlikely.
The council is under pressure because the mandate of the 300-strong UN observer force in Syria expires on Friday and it must decide by then whether to extend it.
The unarmed observers were authorised for 90 days to monitor a ceasefire and implementation of Annan's six-point peace plan.