UN envoy to Syria, Staffan de Mistura, says the city "may be totally destroyed" if the conflict is not resolved soon.
Opposition forces will face their "inevitable fate" if they choose to stay in eastern Aleppo, the Syrian regime warned late on Wednesday. Any residents who do not leave the city will also be affected.
Following the failure of the US-Russia brokered ceasefire, the regime launched a Moscow-backed offensive to take the city. With the use of heavy aerial bombardment the forces encircled Aleppo's opposition-held eastern sector.
Both countries have drawn international criticism for targeting opposition-held hospitals and civilian areas, resulting in dozens of casualties.
"In a maximum of two months, two and a half months, the city of eastern Aleppo at this rate may be totally destroyed," the United Nations Special Envoy for Syria Staffan de Mistura said on Thursday.
De Mistura said history would judge Syria and Russia if they used the presence of about 900 former Nusra Front fighters as an "easy alibi" to destroy the besieged area, killing thousands of the 275,000 citizens, 100,000 of whom are children.
The UN envoy said he is willing to go to eastern Aleppo and escort up to 1,000 "Islamist fighters" out of the city if it will lead to a halt to the bombardment by Russian and Syrian forces.
The regime claims it asked the opposition to leave the area to improve conditions for civilians that it says are used as "human shields."
On Wednesday, the regime said they reduced air strikes on Aleppo to allow civilians to get to safer areas.
But the regime's call did not go beyond an expulsion, as it failed to ensure a safe corridor, or shelters for those who remain in the war-battered city.
UN rights chief Zeid Ra'ad Al Hussein on Tuesday warned Russia that air strikes on civilian targets in Aleppo "may amount to war crimes," adding that 100 children had been killed in 10 days.
War monitors reported hundreds of civilian casualties in the area since the offensive on the city began on Sept. 19.
The Britain-based Syrian Observatory for Human Rights (SOHR) said Syrian regime forces seized around half of a key opposition-held neighbourhood in Aleppo on Thursday in a new advance against the opposition.