Ban Ki-moon told an emergency meeting he had received "credible reports" of civilians killed by bombings and executions
The US ambassador to the UN has lashed out at the Syrian government, and its allies Russia and Iran, saying they bear the responsibility for the "carnage" in Aleppo.
Speaking during an emergency UN Security Council meeting, Samantha Power used strong language as she addressed her counterparts directly.
"To the Assad regime, Russia and Iran, three member states behind the conquest of and carnage in Aleppo: you bear responsibility for these atrocities," she said.
"Is there literally nothing that can shame you?
"Is there no act of barbarism against civilians, no execution of a child that gets under your skin?"
The UN meeting was held as a ceasefire and evacuation agreement was brokered by Russia and Turkey to end years of fighting in the city.
The Syrian forces have been making gains in the rebel-held eastern parts of the Aleppo for weeks, and were close to wrestling full control of what was once Syria's largest city.
However, the evacuations were being delayed and may not start until Thursday. There have also been reports of the sound of bombings in the city despite the ceasefire.
The UN said it was "not involved" in plans to evacuate fighters and civilians from eastern Aleppo, but it was ready to help with any evacuation.
UN secretary-general Ban Ki-moon told the emergency meeting he had received "credible reports" of civilians killed by intense bombing and summary executions by pro-government forces.
UN officials had said earlier that civilians were being shot "on the spot" and in their homes.
"It is your noose," Ms Power said.
"Three member states of the UN contributing to a noose around civilians. It should shame you. Instead, by all appearances, it is emboldening you. You are plotting your next assault. Are you truly incapable of shame?"
In a tense climate, where insults often replaced diplomacy, the Russian ambassador said Ms Power was behaving "like Mother Theresa" and accused the US, UK and France of distributing fake news about the crisis.
Russia, Bashar al Assad's closest ally, has vetoed the last six UN resolutions aimed at easing the dire situation in Aleppo.
Syria's UN ambassador hailed "the liberation of Aleppo" from "terrorists" who controlled the east of the city.
Bashar Jaafari told the meeting that he "firmly and 100%" rejected allegations that Syria has been committing "acts of revenge, summary executions and terror" against civilians.
France's UN ambassador Francois Delattre, whose country had requested the emergency meeting along with the US, said "war crimes and potentially crimes against humanity are committed on a daily basis in Aleppo."