Syria was due on Thursday to announce whether it will accept a ceasefire during a Muslim holiday this weekend, but rebels and Washington remained wary of any commitment from Damascus.
Russia backed up peace envoy Lakhdar Brahimi's assertion on Wednesday that the Damascus regime was ready to observe a truce during the four-day holiday of Eid al-Adha that begins on Friday, while rebels said they would cease fire if regime troops did so first.
A truce, if observed by both sides, would mark a significant breakthrough in the 19-month conflict in which more than 34,000 people have died, according to figures of the Syrian Observatory for Human Rights.
"The Syrian government has agreed to a ceasefire" during Eid, Brahimi said in Cairo, adding that Damascus would later make an official announcement.
The foreign ministry said a final decision on a ceasefire would be taken on Thursday.
"The army command is studying the cessation of military operations during the Eid holiday, and the final decision will be taken tomorrow," a statement said.
Brahimi said "most" rebel leaders contacted said they would observe a truce.
"If we succeed with this modest initiative, a longer ceasefire can be built," he said Wednesday.
Russia's UN envoy Vitaly Churkin said his government had had "indications that they (the Syrian government) are accepting the proposal of Mr Brahimi".
Brahimi has been urging such a ceasefire initiative for weeks, as the violence escalated.
The Syrian Observatory for Human Rights said at least 142 people were killed across Syria on Wednesday, 20 of them civilians including women and children massacred in Douma, a rebel-held town near Damascus.
Brahimi told the UN Security Council a truce would be a "small step" and he was unsure if it would hold, diplomats told AFP.
He appealed for unanimous support, warning the 15-nation council that a new failure among its divided members would cause the 19-month civil war to spread.
The Security Council is bitterly divided over the conflict, with Western nations pressing for action against Assad's regime while Russia and China consistently resist such moves.
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However, it did agree to back Brahimi's ceasefire call and also urged Syria's neighbours to "use their influence" on the parties to push for an end to the conflict.
Susan Rice, US ambassador to the UN, expressed doubts about Syria's commitment to a truce, in comments released after Brahimi's briefing.
"Many are duly sceptical about prospects for even a temporary ceasefire, given Assad's record of broken promises," she said.
The rebel Free Syrian Army said it was for Damascus to stop fighting first.
"The FSA will stop firing if the regime stops," military council chief General Mustafa al-Sheikh told AFP by telephone.
However, he added: "It is impossible that the regime will implement the truce, even if it says it will."
The jihadist militant Al-Nusra Front, which on Wednesday joined the FSA in an assault on Wadi Deif army base in the northwestern province of Idlib, flatly refused to lay down its weapons, denouncing the truce as a "trick".
"There is no truce between us and this transgressing regime that is shedding the blood of Muslims," it said in an Internet statement, quoted by SITE Intelligence Group.
"We aren't amongst those who accept to play such dirty games," said the group, which has claimed suicide bombings and clashed with the Syrian army.
Brahimi said he wanted the cessation to help create political space for dialogue and for aid to flow in, particularly to the cities of Aleppo, Homs and Idlib.
He told the council it was essential that he receive "unambiguous" support for his efforts, warning: "Another failure would lead to the worsening of the conflict and extension to other countries."
In Moscow, Russia's top general, Nikolai Makarov, said the rebels had secured shoulder-launched missile systems capable of shooting down aircraft, including US-made Stingers.
US State Department spokeswoman Victoria Nuland denied the claim.
"We have provided no Stingers of any kind to Syria, nor will we," she said. "If the Russian Federation has evidence of Stingers in the hands of the opposition, we'd like to see it."