The Syrian regime pressed its assault on rebel-held areas of Aleppo on Saturday ahead of a showdown at the United Nations between Western governments and its ally Russia.
German Foreign Minister Frank-Walter Steinmeier warned that the mounting tensions between Washington and Moscow over the conflict had created a situation "more dangerous" than the Cold War.
US Secretary of State John Kerry made clear his anger at the Syrian army's Russian-backed onslaught in the battleground second city, saying that its bombing of civilians could amount to a war crime.
The UN Security Council is to vote on two rival resolutions on the fighting, one drafted by France calling for an end to air strikes and a second by Russia that calls for a ceasefire but makes no mention of halting the bombings.
Moscow has said it will veto the French draft.
Its ally Damascus has made significant advances in its renewed two-week-old offensive in Aleppo, seizing territory to the north and pushing back the front line in the city centre which had remained largely static since the rebels captured eastern districts in 2012.
But it has come at a heavy human cost. Since a US- and Russian-brokered ceasefire collapsed on September 19, at least 305 civilians have been killed in rebel-held areas, 57 them children, according to the Syrian Observatory for Human Rights.
The Britain-based monitoring group said that government forces were making further advances on Saturday ahead of the Security Council session.
"The battle is unfolding in the centre, particularly in the Bustan al-Basha district where the army is advancing, in Sheikh Said in the south, and on the northern outskirts where the regime has taken the Uwaija neighbourhood," Observatory chief Rami Abdel Rahman told AFP.
Russia is expected to use its veto to block the French-drafted resolution backed by the United States and Britain that also calls for a halt to all military flights over Aleppo.
The council will then move to a second vote on the Russian-drafted text.
It "urges immediate implementation of the cessation of hostilities, in particular in Aleppo" and demands that all parties allow deliveries of humanitarian aid."
British ambassador Matthew Rycroft dismissed the Russian draft as a political ploy.
"This is a cynical attempt to divert attention away from the need to stop the bombing of Aleppo," he said.
Council members have been holding negotiations for the past week on the French proposal for an end to the bombing of Aleppo, access for aid deliveries and a ban on military flights over the city.
But Russian ambassador Vitaly Churkin said the measure was "hastily put together" and suggested it was "not designed to make progress... but to cause a Russian veto."
"I cannot possibly see how we can let this resolution pass," he told reporters when asked whether he would use the veto powers Russia wields as one of the council's five permanent members.
A Security Council diplomat, who asked not to be named, said the Russian "resolution on the surface looks like a lot of constructive language that draws from previous resolutions and the French draft, but the key point is that it does not call for an end to the aerial bombardment."
He said the "vast majority" of council members want "an immediate end to the indiscriminate bombing of civilians in Aleppo."
Kerry made clear his anger, declaring: "These are acts that beg for an appropriate investigation -- war crimes."
The German foreign minister said that tensions between Washington and Moscow were now worse than during the Cold War.
"It's a fallacy to think that this is like the Cold War," Steinmeir said in an interview published by Bild newspaper on Saturday.
"The current times are different and more dangerous."
The Security Council session begins at 1900 GMT. If Russia blocks the draft, it will be its fifth veto of a UN resolution on Syria.