Syria and most rebel chiefs have agreed to a truce this week, peace envoy Lakhdar Brahimi said, but the main armed opposition group and Washington remained wary of any commitment from Damascus.
"The Syrian government has agreed to a ceasefire" during Eid, Brahimi said, adding that "most" rebel leaders contacted said they would observe it, boosting hopes of a breakthrough in the conflict.
"If we succeed with this modest initiative, a longer ceasefire can be built," he said Wednesday.
Syria said it was studying the UN-Arab League envoy's initiative for a four-day ceasefire during the Muslim holiday of Eid al-Adha and would announce its decision Thursday, the day before it is supposed to take effect.
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, but as the date for its implementation fast approaches there has been no let up in the deadly violence.
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.
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.
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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.
A truce would be the most important breakthrough since the conflict spread from demonstrations back in March 2011 to engulf the entire country. Brahimi's predecessor Kofi Annan announced a short-lived ceasefire in April.
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 Syria the violence continued.
The bodies of some 20 civilians, including women and children, were found in a building in Douma east of Damascus, said the Observatory.
The army blamed rebels for the Douma killings and put the death toll at 25.
Syrian warplanes hammered the rebel-held town of Maaret al-Numan near the Wadi Deif army base along the highway linking Damascus to northern metropolis Aleppo, a key supply route for troops, said the Observatory.
Air raids also struck Maaret Shamirin village in the northern province of Raqa bordering Turkey, killing five members of one family, it added.
A car bomb in Raqa killed at least eight soldiers, while another in the southern Damascus district of Daf al-Shuk, claimed a further eight lives. State television gave a lower toll of six for the southern neighbourhood attack.
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 dismissed any suggestion that they had supplied the missiles.
"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."