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's government not to destroy the city for the sake of eliminating militants.
Troops have been besieging the east, where 275,000 people live, for a month.
The bottom line is, in a maximum of two months, two and a half months the city of eastern Aleppo at this rate may be totally destroyed," Mr de Mistura told a news conference in Geneva.
Thousands of Syrian civilians, not terrorists, will be killed and many of them wounded.
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