At the UN Security Council emergency meeting, the US accused Russia of "barbarism" in Syria. Warplanes supporting Syrian government forces again pounded Aleppo where water supplies have been destroyed.
"Instead of pursuing peace, Russia and Syrian President Bashar al-Assad make war," Samantha Power, US ambassador to the United Nations, told the Security Council members on Sunday.
"Even now, we will continue to look for any way possible to restore the cessation of hostilities, but it is common sense: a one-sided cessation of hostilities cannot hold," she added.
The US and Russia agreed on a ceasefire plan on September 12, but the truce looks increasingly shaky as the Russian ambassador to the UN Vitaly Churkin told the council on Sunday: "In Syria hundreds of armed groups are being armed, the territory of the country is being bombed indiscriminately and bringing a peace is almost an impossible task now because of this."
Residents and rebels say thousands have been killed in the new strikes on Aleppo where water supplies have been destroyed. "Instead of pursuing peace, Russia and Assad make war. 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 told the Council on Sunday.
The French and British foreign ministers also directed their verbal attacks at Russia, saying it could be guilty of war crimes.
The fierce fighting in Aleppo and the bombing of an aid convoy last week effectively collapsed the ceasefire deal. Since then, Moscow and Washington, which back opposing sides in the Syrian civil war, have been accusing each other of violating the peace deal.
War crime allegation
British Foreign Minister Boris Johnson said "whether the bombing of the aid convoy was done with the knowledge that the targets were civilian, and would therefore constitute a war crime," Johnson said, adding that the ongoing bombing in Aleppo was "absolutely barbaric."
Francois Delattre, French ambassador to the UN, called on Russia to relinquish support for President Assad.
"Does Russia think that it can install confidence and trust with its partners by simultaneously negotiating a cessation of hostilities and supporting the regime, which is bombing Aleppo" asked Dealattre.
Despite the war of words, UN Syria mediator Staffan de Mistura appealed to the Council meeting to come up with a way to enforce a ceasefire: "I am still convinced that we can turn the course of
events," he said, adding that he would not stop trying to bring peace in Syria.
'Sustained and intense bombing' in Aleppo
The UN has been particularly concerned about the intense fighting in Aleppo. UN Secretary-General Ban Ki-moon urged the world powers to end the "nightmare" in Syria by putting aside their differences.
"What excuse is there for anything less than determined action to stop the mayhem? How much longer will all those with influence allow such cruelty to continue?" he asked. "I urge all involved to work harder for an end to the nightmare."
Thousands of civilians remained trapped in Aleppo on Sunday, three days into a Syrian army-led offensive aimed at seizing rebel-held areas of the country's former economic capital. "The Assad regime and with direct participation of its ally Russia and Iranian militias has escalated its criminal and vicious attack on our people in Aleppo employing a scorched earth policy to destroy the city and uproot its people," a statement signed by 30 mainstream rebel groups said on Sunday.
The Syrian conflict erupted in 2011, when government forces launched a violent crackdown against pro-democracy protesters calling for President Assad to step down.
More than 200,000 people have been killed and half the population displaced since the conflict erupted five years ago.