"We are gravely concerned about the safety of civilians in Aleppo -- those who remain in opposition-controlled areas as well as those who have fled to areas under government control," said UN High Commissioner for Human Rights spokesman, Rupert Colville.
Speaking in Geneva, Colville said people who remained in opposition-controlled districts were being squeezed into ever-shrinking areas, as regime forces and their allies continued their advance and concentrated their firepower in recent days.
He said: "We believe there may currently be around 100,000 civilians in areas under the control of armed opposition groups in eastern Aleppo, with another 30,000 believed to have fled to areas under government control."
"We are also concerned about the approximately 500 medical cases involving people in need of urgent evacuation from opposition-held areas," Colville said.
"We have received very worrying allegations that hundreds of men have gone missing after crossing into government-controlled areas," the spokesman added.
Indicating that there are also approximately 150 activists inside opposition-controlled Aleppo who fear being detained by regime forces if they attempt to leave, Colville said: "Given the terrible record of arbitrary detention, torture and enforced disappearances by the Syrian government, we are of course deeply concerned about the fate of these individuals."