United Nations, The US accused Moscow of "barbarism" over the worsening carnage in Aleppo, as Syrian and Russian warplanes pounded the city in one of the heaviest bombing raids of the five-year war.
At an emergency meeting of the UN Security Council to demand Russia rein in its ally, Syrian President Bashar al-Assad, and halt intense air strikes, Moscow and Damascus were repeatedly accused of war crimes.
"What Russia is sponsoring and doing is not counter-terrorism. It is barbarism," US Ambassador Samantha Power said at the Sunday session.
Some 124 people, mostly civilians, have died since bunker-busting bombs and sophisticated weaponry were unleashed on residential areas in rebel-held eastern Aleppo after the army Thursday launched an operation to take it.
"It is difficult to deny that Russia is partnering with the Syrian regime to carry out war crimes," said British Ambassador Matthew Rycroft, adding that the high-tech weaponry had inflicted "a new hell" on war-weary Syrians.
UN Secretary-General Ban Ki-moon has also warned the use of advanced weaponry against civilians could amount to war crimes, and French Ambassador Francois Delattre said the atrocities must not go unpunished.
Britain, France and the United States had called for the urgent talks after days of intense diplomatic efforts to salvage a US-Russian ceasefire deal ended in failure at the weekend.
Ban called on world powers to "work harder for an end to the nightmare" in Syria that has left more than 300,000 people dead and driven millions from their homes.
To protest the attacks in Aleppo, the US, French and British ambassadors walked out of the Security Council chamber as the Syrian ambassador delivered his remarks.
Russia's Ambassador Vitaly Churkin conceded that the surge in violence over the past days meant that "bringing a peace is almost an impossible task now."
Churkin again laid blame for the failed diplomacy with the United States, accusing Washington of being unable to convince armed opposition groups that it backs on the ground to distance themselves from the Al-Qaeda-linked Nusra Front and abide by the ceasefire.
A US-Russian ceasefire deal that would have charted a way forward towards peace talks was broken by the "sabotage by the moderate opposition", he asserted.
Churkin however said that reviving the ceasefire was still a goal that Moscow could pursue if it was part of a "collective" effort on all sides.
Despite the recriminations, UN envoy Staffan de Mistura said a "tiny window of opportunity ... still exists" for Russia and the United States to help Syria.