(Rewrites throughout after Russian ambassador tells U.N. Security Council siege ended; adds United States, Britain, U.N. chief quotes)
By Michelle Nichols
UNITED NATIONS, Dec 13 (Reuters) - A Syrian government military offensive in Aleppo, backed by Russia and Iran, was over, Russia's U.N. envoy said on Tuesday as the United States described the violence in the besieged city as "modern evil."
Ambassador Vitaly Churkin said an agreement had been struck for rebels to evacuate the north-western city and he said civilians would be unharmed, despite western and U.N. accusations of the intentional killing of civilians.
"Over the last hour we have received information that the military activities in east Aleppo have stopped, it has stopped," Churkin told a heated U.N. Security Council meeting called by France and Britain. "The Syrian government has established control over east Aleppo."
A surrender or withdrawal of the rebels from Aleppo would deliver Syrian President Bashar al-Assad his biggest battlefield victory in the nearly six year conflict.
The United Nations said on Tuesday it had reports that Syrian soldiers and allied Iraqi fighters had summarily shot dead 82 civilians in recaptured districts of Aleppo, which was Syria's largest city before the civil war began in 2011.
"They have gone from siege to slaughter," British U.N. Ambassador Matthew Rycroft told the 15-member council.
U.N. Secretary-General Ban Ki-moon, in his briefing to the council, called on the Syrian government, Russia and Iran to urgently allow civilians to escape Aleppo.
"There was an abundance of early warning given to this council regarding the situation in Aleppo," Ban said. "We have collectively failed the people of Syria ... History will not easily absolve us."
Churkin told reporters that Russian military personnel had not seen "any abuses of international humanitarian law." The Syrian army has denied carrying out killings or torture among those captured.
The U.S. Ambassador to the United Nations, Samantha Power, said the Syrian government, Russia and Iran would be responsible for atrocities committed in Aleppo.
"By rejecting U.N./ICRC (International Committee of the Red Cross) evacuation efforts you are signaling to those militia who are massacring innocents to keep doing what they are doing," Power said.
"Aleppo will join the ranks of those events in world history that define modern evil, that stain our conscience decades later - Halabja, Rwanda, Srebrenica and now Aleppo," she said.
A crackdown by Assad on pro-democracy protesters in 2011 led to civil war and Islamic State militants have used the chaos to seize territory in Syria and Iraq. Half of Syria's 22 million people have been uprooted and more than 400,000 killed.
U.N. Syria mediator Staffan de Mistura told reporters after the meeting that the United Nations wants its representatives to be allowed to be there when civilians are evacuated and opposition fighters withdraw.
De Mistura said there were an estimated 50,000 civilians still in rebel-held territory of Aleppo, along with 1,500 opposition fighters of which he said about 30 percent belong to jihadist group Nusra Front.
(Reporting by Michelle Nichols; editing by Grant McCool)
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