The United States accused Russia of "barbarism" and war crimes in Syria as Moscow's airstrikes over Aleppo pushed a humanitarian crisis there to new depths.
The nations sparred verbally at an emergency UN Security Council meeting called to demand that Russia rein in its ally, Syrian President Bashar al-Assad, and halt the blistering attacks on Syria's second city.
"Instead of pursuing peace, Russia and Assad make war," said Samantha Power, the US Ambassador to the United Nations. "What Russia is sponsoring and doing is not counterterrorism. It is barbarism."
A September 9 ceasefire deal guaranteed by the United States and Russia was smashed a week ago by Russian and Syrian airstrikes on a UN aid convoy. Despite frantic diplomacy to get that truce back on track, today's Security Council meeting suggested that divisions between the two sides were deepening.
"Instead of helping get lifesaving aid to civilians, Russia and Assad are bombing the humanitarian convoys, hospitals and first responders who are trying desperately to keep people alive," Power said.
But Russia's representative, Vitaly Churkin, instead blamed his American counterparts for the return to fighting and insisted that Assad's forces had shown "admirable restraint".
"I just need to explain what working with our American colleagues is like," he said, telling the 15-member council that Washington had failed to rein in violence by the rebel forces it backs in Syria. "Bringing a peace is almost an impossible task now," he said.
As the war of words unfolded in New York, Aleppo's rebel-held eastern neighbourhoods were being shaken by the most ferocious aerial attacks there in recent memory. A provisional death toll provided by local nongovernmental organisations suggested that at least 85 people had been killed there today.
"This is the worst day," said Abdulkafi al-Hamdo, an English teacher living close to the city's front line. "The people here are psychologically broken."
The latest attacks have appeared to target what resources the eastern neighbourhoods have left. Branches of at least three rescue teams have been hit by airstrikes, and firetrucks and ambulances have been damaged or destroyed.
Hamdo said that some rescue teams no longer had enough ambulances to reach families suffocating under the rubble of their homes.
For Assad, Aleppo represents an important prize that would expand government control in the north and deprive opposition groups of one of their last strongholds.
Home to an estimated 275,000 people, east Aleppo's rebel-held neighbourhoods have been under near-continuous siege since government troops encircled the area in mid-July.
Residents say fuel and medical supplies are low, forcing doctors to turn off oxygen machines and operate by the light of their cellphones.
"We've never seen anything this bad," Maher Saqqur, a Syrian neurosurgeon, said, speaking from a Canadian clinic where he consults with Aleppo doctors via Skype.
"The doctors can do nothing but triage on the floor, and still the bodies keep coming. They don't even have time to take a sip of water. We're seeing massacres every hour," he said.
Hundreds of civilians have been killed in east Aleppo over the past week. According to eyewitnesses, the arsenal raining down has included white phosphorus, cluster munitions, barrel bombs and "bunker busters" - munitions so powerful that they can reach the basements where civilians try to shelter from attacks.
UN special envoy Staffan de Mistura described the attacks as "unprecedented in scale and type". He said his team had been notified of "fireballs of such intensity that they light up the pitch darkness as if it was daylight".
His words drew immediate condemnation from Iran - a key force behind the Assad Government's survival - which insisted that pro-Assad forces were not using internationally banned weaponry.
According to Save the Children, half of the casualties being pulled from the rubble or treated in hospitals are children. Saqqur said that a growing number of his infant patients were being brought to the hospital with the shrapnel of cluster munitions penetrating their brains.
If the Syrian Government is intent on taking Aleppo, de Mistura warned, it will face a "grinding" fight that destroys what is left of the city without any guarantee of victory.
"A so-called military solution is impossible, including in Aleppo," he said, urging the United States and Russia to go "that extra mile" and save the cessation of hostilities agreement "at the 11th hour".
But as darkness fell on Aleppo and the bombs continued, civilians there voiced deep scepticism over the prospects of an internationally brokered peace. "The best thing the UN Security Council can do is stop their talking," Hamdo said. "It makes no difference here."