World powers have pressed Russia to stop bombing around Aleppo in support of a Syrian government offensive to recapture the city.
The move comes while a Western official said Russia has presented a proposal envisaging a truce in three weeks' time.
US Secretary of State John Kerry is pushing for a ceasefire and more aid access to Aleppo, where rebel-held areas are being cut off and the United Nations has warned a new humanitarian disaster could be on the way.
Aid workers say the water supply to Aleppo, still home to two million people, is no longer functioning.
Mr Kerry is hoping for agreement at a meeting in Munich tomorrow between Russia, the United States, Saudi Arabia, Iran and other powers, aimed at trying to revive peace negotiations that foundered earlier this month.
Syrian officials have indicated no plans to ease up the war effort.
A Syrian military source said the battle for Aleppo, a major prize in a war which has killed a quarter of a million people, would continue in "all directions".
Syrian Information Minister Omran al-Zoubi said the government expected a tough but relatively short battle to return the city to state control.
"I do not expect the battle of Aleppo to go on long," he told Reuters in Damascus.
Russia proposes ceasefire to begin in March
A Western official said Russia had made a proposal to begin a ceasefire in Syria on 1 March, but that the US has concerns about parts of it and no agreement had been reached.
In Washington, a state department envoy told Congress the US needs to consider options in case the diplomatic push does not succeed.
Asked how soon a ceasefire could be put in place, a Russian diplomat who declined to be identified said: "Maybe March, I think so."
At a closed-door meeting of the 15-member UN SecurityCouncil, several members pressed Russia to end the Aleppo bombing sooner.
Footage has been released purportedly showing Russian airstrikes on the Syrian town of Tell Rifaat in Aleppohttps://t.co/MshqWmWeYc
— RTÉ News (@rtenews) February 10, 2016
Over 500 dead in Syria regime's Aleppo offensive
More than 500 people, including dozens of civilians, have been killed since a major Russian-backed regime offensive in Syria's Aleppo province began this month, a monitor said today.
The Syrian Observatory for Human Rights, which relies on a network of sources for its figures, said the toll of 506 included 23 children killed in Russian air strikes on Aleppo city and its surroundings since the operation was launched on 1 February.
"At least 143 pro-regime fighters, 274 rebels and foreign jihadists, and 89 civilians were killed from 1 February until Tuesday night," Observatory director Rami Abdel Rahman told AFP.
He said that 14 Iranian troops and at least three fighters from Lebanon's Hezbollah movement had died fighting alongside the forces of Syrian President Bashar al-Assad.
Mr Abdel Rahman added that more than 100 foreign jihadists, including members of Syria's Al-Qaeda affiliate, had died since the offensive began.
Fighting today raged around Tamura, north of Aleppo city, with intense Russian air raids on several nearby villages, the Observatory said.
Pro-regime forces have made a series of gains this month in Aleppo province, severing rebel supply lines and prompting tens of thousands to flee towards the Turkish border.
The UN has warned 300,000 people in eastern Aleppo city could be cut off from humanitarian aid if government forces encircle the area, a tactic used by the regime to devastating effect against other former rebel bastions.
