UPDATED 5.18pm: A Syrian government military offensive in Aleppo, backed by Russia and Iran, is over, Russia's UN envoy has said as the United States described the violence in the besieged city as "modern evil."

LISTEN ABOVE: Relief Aid's Mike Seawright spoke to Larry Williams about the current situation in Aleppo

Ambassador Vitaly Churkin said on Tuesday that an agreement had been struck for rebels to evacuate the north-western city and civilians would be unharmed, despite western and UN accusations that civilians had been intentionally killed.

"Over the last hour we have received information that the military activities in east Aleppo have stopped, it has stopped," Churkin told a heated UN 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 UN Ambassador Matthew Rycroft told the 15-member council.

UN 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 US Ambassador to the United Nations, Samantha Power, said the Syrian government, Russia and Iran would be responsible for atrocities committed in Aleppo.

"By rejecting UN/ICRC (International Committee of the Red Cross) evacuation efforts you are signalling 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.

UN 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 per cent belong to jihadist group Nusra Front.

Executive Director of Relief Aid Mike Seawright told Larry Williams there are concerns about what will happen next among citizens, particularly young men.

"And they will be, at the very least, detained for military service and therefore forced to fight against their own people, for a regime that they feel has systematically destroyed their country."

Mr Seawright told Larry Williams Relief Aid is now focusing on two main objectives.

"That is to gain safe passage for our staff out of the city, so that they are not detained and possibly tortured and killed in detention. Also the delivery of humanitarian assistance, which is absolutely needed now."