Staffan de Mistura told reporters that he was prepared to personally accompany al-Qaeda-linked jihadists out of the city if it would stop the fighting.
He also appealed to Russia and Syria not to destroy the city for the sake of eliminating militants.
The UN says 275,000 civilians are trapped in the war-torn east of Aleppo.
"The bottom line is, in a maximum of two months... the city of eastern Aleppo at this rate may be totally destroyed," Mr de Mistura told a news conference in Geneva.
