The vote, expected later on Wednesday, gained new urgency as fierce fighting erupted across Damascus and rebel fighters at the forefront of the 16-month-old uprising claimed the battle to "liberate" the capital had begun.
Syria's main ally Russia – which has vowed to veto a Western-backed proposal calling for sanctions – saw its own proposal rejected Tuesday by Britain, France, the US, Germany and Portugal, diplomats said.
"Barring a last minute surprise, we should now go for a vote on Wednesday and we expect a veto by Russia and China," the UN envoy of a Western nation said on Tuesday on condition of anonymity because talks were still going on.
Representatives of the Syrian National Council (SNC) – an umbrella opposition group – met ambassadors from the 15-nation Security Council, including Russian envoy Vitaly Churkin, to press them to back sanctions.
The SNC warned that it would seek alternative ways to defend civilians if the Security Council does not threaten sanctions over the government's brutal crackdown in a conflict activists say has killed more than 17 000 people.
Basma Kodmani, the SNC's head of foreign affairs, said the Western-backed resolution was "a very last chance for breathing life" into the repeatedly violated peace plan of UN-Arab League envoy Kofi Annan.
"Should the current attempts fail, the [SNC] will explore other alternatives with international and regional friends in order to provide humanitarian protection to the Syrian people," Kodmani told a press conference.
Russia and China have already vetoed two UN resolutions that hinted at sanctions. A third veto would be a "blank check to continue the violence, to continue crushing the population," said Kodmani.
"I think Russia gets the message very clearly," she said.
"We have made clear to our Russian interlocutors, both in Moscow last week and today, that they have every interest in turning the page of the Assad regime with the Syrian people."
SNC leaders met top Russian officials in Moscow last week.
The Syrian opposition and its Western supporters had hoped for some sign of a breakthrough from a Moscow meeting between Annan and President Vladimir Putin, but none came.
Russia has also proposed a new version of its resolution on the future of the UN Supervision Mission in Syria (UNSMIS). Russia was trying to be "accommodating", said its deputy UN ambassador Alexander Pankin.
"There is nothing new in it," the Western envoy responded.
The Western-backed resolution calls on the Security Council to consider sanctions under Chapter VII of the UN Charter if Assad's forces do not withdraw heavy weapons from Syrian cities within 10 days.
The UNSMIS mandate ends on Friday and the Western nations' proposal would only renew the mission for 45 days.
The Russian draft would renew the mission for three months, but would not back it up with international action.
If Russia introduces its resolution for a vote, Western members are confident that at least seven of the 15 council members will abstain, which would see it fail. A resolution needs at least nine votes with no veto to pass.
Russia's Foreign Minister Sergei Lavrov has accused the West of using "blackmail" by making sanctions a condition of renewing the UNSMIS mission.
But this week Lavrov said he saw "no reason why we cannot also agree at the UN Security Council. We are ready for this."
Special envoy Annan and UN leader Ban Ki-moon have both called for the Security Council to impose "consequences" if Assad and the Syrian opposition fail to carry out Annan's peace plan.
Ban met with Chinese President Hu Jintao on Wednesday in a bid to convince Beijing to back tougher action against Assad.
Russia insists that diplomatic pressure is enough. According to diplomats, Putin spoke with Hu at the weekend and the two agreed to oppose sanctions. – AFP