Russia's United Nations Ambassador Vitaly Churkin says the Syrian government's military offensive in Aleppo, backed by Russia and Iran, is over and that an agreement has been struck for rebel fighters for the safe evacuation of the city.

Churkin told a UN Security Council meeting called by France and Britain on December 13 that civilians would not be harmed under the deal.

But the United States has described the violence in Aleppo as a "modern evil," and the United Nations, along with several Western governments, has raised accusations that Syrian government forces are intentionally killing civilians.

Churkin told the heated Security Council meeting that "the military activities in east Aleppo have stopped" and that the Syrian government on December 13 had "established control over east Aleppo."

Britain's UN Ambassador Matthew Rycroft told the 15-member Security Council that Syrian government forces had "gone from siege to slaughter" in formerly rebel-held neighborhoods of eastern Aleppo.

The United Nations says it has received reports of pro-government forces in Syria entering homes in eastern Aleppo and summarily shooting civilians dead "on the spot."

'Meltdown Of Humanity'

The UN human rights office said on December 13 that it has reliable evidence that in four neighborhoods 82 civilians, including 11 women and 13 children, had been shot.

Spokesman Rupert Colville said the atrocities were committed in recent days, adding that there could be "many more" execution-style killings.

UN humanitarian spokesman Jens Laerke said it looked like "a complete meltdown of humanity in Aleppo."

Rebels who have held eastern Aleppo for four years are on the brink of defeat.

Thousands of civilians trapped in the remaining rebel-held neighborhoods "have literally nowhere safe to run," the International Committee of the Red Cross has said, urging the fighters to do all they can to protect and spare their lives.

Syrian President Bashar al-Assad's ally, Russia, which has rejected calls for a humanitarian cease-fire in Aleppo, earlier said atrocities were "actually being committed by terrorist groups," meaning rebel forces.

But the UN humanitarian adviser on Syria, Jan Egeland, said on Twitter on December 12 that both the Syrian and Russian governments were "accountable for any and all atrocities that the victorious militias in Aleppo are now committing."

Russian Foreign Minister Sergei Lavrov, speaking to reporters in Serbia on December 13, said Moscow was "tired of hearing this whining from our American colleagues in the current administration that we need to immediately halt military action."

Lavrov also accused Washington of doing nothing to separate moderate rebels from "terrorists" in the city.

Russia has given Assad crucial military and diplomatic backing throughout the conflict in Syria, which began with a forceful government crackdown on pro-democracy protesters in 2011 and erupted into a civil war that has killed more than 250,000 people.

Moscow helped turn the tide of the war in the government's favor with a major campaign of air strikes that began in September 2015 and which Western governments say has mainly targeted rebels rather than Islamic State militants.

With reporting by AP, AFP, Reuters, and TASS