Washington has demanded that Russia restrain its Syrian ally as the United States and the United Nations condemned an airstrike on a hospital in Aleppo.
UN officials also voiced alarm at the "catastrophic deterioration" of the situation in Syria and appealed to world powers to salvage a February 27 truce.
But in Aleppo yesterday, fighting between rebels and regime forces killed 53 civilians - the highest toll for a single day in a week of violence that has cost more than 200 lives, according to a monitor.
Meanwhile, the Syrian army was poised to launch an offensive against rebels who control part of the northern city.
US Secretary of State John Kerry expressed outrage over Thursday's airstrike that hit Al-Quds hospital in Aleppo's rebel-held Sukkari neighbourhood.
He said it appeared to be "a deliberate strike on a known medical facility" and said Russia, which backs Syrian President Bashar al-Assad, must restrain the Damascus regime.
"Russia has an urgent responsibility to press the regime to fulfil its commitments under UNSCR 2254, including in particular to stop attacking civilians, medical facilities and first responders, and to abide fully by the cessation of hostilities."
UN Secretary-General Ban Ki Moon said attacks that target civilians were "inexcusable" violations of humanitarian law.
"There must be accountability for these crimes," he said.
The city's last remaining paediatrician and three children were among the 20 people killed in the airstrike on the Al-Quds hospital, which was supported by Doctors Without Borders (MSF).
A civil defence group known as the White Helmets told AFP 30 people were killed in the strike on the hospital and a nearby block of flats.
The Aleppo violence has raised fears for the ceasefire in other areas of Syria and called into question the future of UN-brokered peace talks in Geneva that have now gone into recess.
UN envoy Staffan de Mistura warned that the hard-won February 27 ceasefire was now "barely alive" and could collapse "any time".
UN aid chief Stephen O'Brien further raised the alarm yesterday in remarks made at the UN Security Council. "We must all be ashamed this is happening on our watch," O'Brien said, urging world powers to salvage the truce. "You must not squander the opportunity presented by talks in Geneva and by the cessation of hostilities to put an end to the massive human suffering in Syria."
And the UN's Jan Egeland, who heads an international humanitarian taskforce for war-ravaged Syria, spoke of a "catastrophic deterioration" of the situation in the country.
"The stakes are so incredibly high," Egeland told reporters in Geneva, and warned "the lifeline to hundreds of thousands, if not millions of people ... may be broken."
Rebel rocket and artillery fire on government-held neighbourhoods yesterday killed 22 civilians, including two children, the Syrian Observatory for Human Rights said.
Regime airstrikes on rebel-held districts, including Bustan al-Qasr, killed 31 civilians, including three children, said the British-based monitor.
An AFP correspondent said every building in sight in the Bustan al-Qasr district had had its windows blown out.
"It is the worst day in Aleppo in five years," said one resident. "The regime did not spare a single neighbourhood."
The pro-government Al-Watan newspaper and a Syrian regime source, meanwhile, said the army was preparing to take the whole of Aleppo.
"Now is the time to launch the battle for the complete liberation of Aleppo," the paper said, adding that it "will not take long to begin, nor to finish".
A regime source told AFP that "the army is preparing a huge operation in the coming days to push the rebels away from the city by encircling it and creating a security zone".
Rebels have controlled eastern districts of Aleppo city since 2012, while western neighbourhoods are held by the regime.
Control of the surrounding province is divided between a myriad of armed groups - jihadists of al-Qaeda and Isis (Islamic State), Kurdish militia and various rebel factions as well as the army.
More than 270,000 people have been killed in Syria and millions forced from their homes since the conflict erupted in 2011.