Reports are coming out of eastern Aleppo of Syrian forces shooting people in their homes and on the streets in the last pockets of the city held by rebels.
The UN and the Red Cross says troops are busting into houses where civillians are holed up, too terrified to leave.
There are reports of more than 100 children trapped in one building under fire.
A UN human rights spokesperson Jens Laerke in Geneva says the situation is grim.
"Yesterday evening we received deeply disturbing reports that numerous bodies were laying in the streets. Residents were unable to retrieve them due to the intense bombardment and their fear of being shot on site."
He says it looks like "a complete meltdown of humanity in Aleppo".
UN human rights office spokesman Rupert Colville says there is intense bombardment and people are being shot on sight."
"Civilians have paid a brutal price during this conflict and we're filled with a deepest foreboding for those who remain on this last hellish corner of opposition held eastern Aleppo.
"While some reportedly managed to flee yesterday some were reportedly caught and killed on the spot and others were arrested."
A Red Cross spokesman in western Aleppo says the crisis has turned into a deep humanitarian catastrophe.
Despite all these reports Syria denies it is committing atrocities in Aleppo.
The Kremlin has also released a strong statement, denying suggestions of war crimes and saying the battle has been successful and humane as regards civilians.
In response to suggestions of summary executions the statement says there has been no opposition in rebel- held areas - only terrorists.