The US ambassador to the UN has accused Russia of "barbarism" over the bombing of the Syrian city of Aleppo.
At an emergency meeting of the UN Security Council, Samantha Power said Russia had told the council outright lies about its conduct in Syria.
She said Russia and the Syrian regime were "laying waste to what is left of an iconic Middle Eastern city".
Russia said Syrian forces were trying to remove terrorists from Aleppo while harming as few citizens as possible.
Ambassador Vitaly Churkin did not say Russian forces were involved.
But he said that bringing peace to Syria was "almost an impossible task now." He also accused opposition armed groups of sabotaging the ceasefire.
The northern city of Aleppo has become a key battleground in Syria's bloody five-year civil war.
Save the Children said on Sunday that humanitarian workers on the ground reported that approximately half of the casualties pulled from the rubble were children.
One hospital told the charity that 43% of the injured they treated on Saturday were children, and a Syrian ambulance crew said more than 50% they picked up in the past 48 hours were children.
Ms Power told the meeting that Russia, which is supporting the regime of Syrian President Bashar al-Assad, had "long had the power to stop this suffering".
She said: "Instead of peace, Russia and Assad make war. Instead of getting life-saving aid to Syrians, Russia and Assad are bombing hospitals and first responders."
She accused Russia of preparing air strikes against eastern Aleppo even at the very moment its Foreign Minister Sergei Lavrov was telling the UN that Russia was committed to a peaceful solution.
Ms Power went on to call on the council to "have the courage to say who is responsible and tell Russia with one voice to stop".
Several representatives at the meeting suggested Russia may have committed a war crime over the bombing of a humanitarian convoy near Aleppo on Monday.
Russia has denied carrying out the attack, which destroyed 18 of 31 aid trucks. It has said that rebel shelling or a US drone were responsible.
Russia has not admitted to bombing Aleppo since the end of the recent ceasefire.
UN special envoy on Syria Staffan de Mistura said at least 213 civilians had been killed since the offensive began, many of them women and children. He said the conflict had reached "new heights of horror".
Hours after the end of a week-long cessation of hostilities on Monday, the Syrian regime declared a new offensive against eastern Aleppo and jets began pounding the area, where 275,000 people are trapped.
Mr Churkin said there was a humanitarian corridor by which residents could leave but it was being blocked by rebels.
Mr de Mistura said reports suggested Russia had used incendiary weapons in Aleppo, which he said "create fireballs of such intensity that they light up the pitch darkness in Aleppo as if it were daylight".
He said that Russia and the Syrian regime were also using so-called bunker busting bombs - designed to penetrate the earth and destroy underground targets - against residential areas.
"The systematic and indiscriminate use of such weapons in areas where civilians are present may amount to a war crime," he said.
Mr de Mistura called for the Council to recommend weekly 48 hour pauses in fighting to ensure that UN aid workers could reach eastern Aleppo.
The UK ambassador accused the Syrian regime of a "sick bloodlust against its people".