UN Security Council powers headed for a showdown over a western-backed resolution that threatens sanctions against President Bashar al-Assad that Russia has vowed to veto when it comes to a vote on Wednesday.

Britain, France, the United States, Germany and Portugal say it is time to step up the pressure on Assad. Russia has branded as "blackmail" a bid to make renewal of the UN mission in Syria conditional on sanctions.

"I made it very clear we are going to vote against this resolution," Russia's UN ambassador Vitaly Churkin told reporters after the latest tense talks between council ambassadors.

A 'no' vote by Russia, one of the five permanent members of the 15-nation council and a key ally of Assad, would be a veto. Russia and China, another permanent member, have twice vetoed resolutions which hinted of sanctions against Assad over the 16-month old conflict in his country.

A rival Russian resolution just proposing to renew the UN mission would fail to get enough votes to pass, US envoy Susan Rice told reporters.

As the 90-day mandate of the UN Supervision Mission in Syria (UNSMIS) ends on Friday, if no resolution is passed by then, it would have to shut down this weekend, diplomats said.

Rice said it would be "immoral" to leave the nearly 300 unarmed observers in Syria if the council was not going to pressure Assad to carry out the peace plan of UN-Arab League envoy Kofi Annan.

Russia's Foreign Minister Sergei Lavrov has accused the West of using "elements of blackmail" by trying to get Moscow to agree to link sanctions to the renewal of the UNSMIS mandate.

Britain, France, the United States, Germany and Portugal have called for a vote on their resolution -- proposing sanctions under Chapter 7 of the UN Charter -- on Wednesday.

Under the resolution, Syria would face sanctions if it does not stop using heavy weapons and withdraw its troops from towns within 10 days of the resolution.

It would extend the UNSMIS mission for 45 days while Russia's proposal would keep it going for three months.

Churkin said the council should concentrate on extending the UNSMIS mandate, noting: "If people want to attach their individual political agendas, it means they do not want this mission to continue."

Rice countered that the rival Russian resolution would not get the required minimum number of votes to pass.

"I don't think there are nine votes for the Russian draft," she said.

The western allies are confident that they can get at least one other country to join them in abstaining, which would avoid the need for a veto.

Rice, who has previously said the United States could block the extension of the UN mission, said that continuing UNSMIS was "questionable" as there is no ceasefire in Syria and no political process.

"Clearly continuing to do the same old thing is not working. The logic of the draft resolution that the British have tabled and which we strongly support is that there needs to be something new. There needs to be pressure applied under Chapter 7," the US envoy said.

Ambassadors are expected to meet again Tuesday, and China's envoy Li Baodong said he was trying to persuade both sides to put back a vote to allow more negotiating time.

The west however appears determined to hold a vote on Wednesday.

"There is clearly overwhelming support for the text," said Mark Lyall Grant, UN envoy for Britain, which took the lead writing the resolution.

"Russia and China still expressed objections to Chapter 7, but when challenged, they were unable to come up with any convincing reasons why," Lyall Grant added.

"Everyone else appeared to be happy with the text as is it is now emerging. So obviously we are happy to have further negotiations, we have scheduled a vote for Wednesday afternoon."

Lyall Grant said that "high level contacts" are being held between the major powers on the sanctions dispute.