"In maximum two months, two and half months the city of eastern Aleppo may be totally destroyed", Staffan de Mistura told reporters in Geneva.
The rebel-held eastern part of Aleppo has been hammered by a Russian-backed government offensive, including multiple attacks on hospitals.
De Mistura noted that the presence of Al-Nusra fighters in the city has been used as a justification by Moscow and Damascus for the continued assault.
The former Al-Nusra Front has recently changed its name to Fateh al-Sham Front following a break with Al-Qaeda, but many still see the two groups as tied.
"If you decide to leave with dignity... I am personally ready to physically accompany you," the UN envoy said.
The UN estimates that 275,000 civilians are under siege in east Aleppo, with aid deliveries all but impossible since government forces seized the last supply route in July.
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