BEIRUT, Lebanon -- The U.N. envoy for Syria, Staffan de Mistura, has urged al Qaeda-linked fighters in the rebel-held eastern parts of the northern Syrian city of Aleppo to leave the area.

De Mistura also suggested Russian and Syrian forces should immediately stop their “aerial bombing” of Aleppo, if the militants from the Fatah al-Sham Front group -- formerly known as the Nusra Front, which has been linked to al Qaeda -- leave the city.

He said if the militants lay down their weapons “in dignity” and leave, he would “personally” accompany them out. De Mistura’s remarks underscore the brutality of the fighting in Aleppo, where a besieged population of 275,000 in the eastern, rebel-held part of the city, is in desperate need of aid. The U.N. considers the Nusra Front a terrorist group.

“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. We are talking about the old city in particular,” de Mistura said to reporters in Geneva on Thursday, according to the Reuters news agency.

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“There is only one thing we are not ready to do: be passive, resign ourselves to another Srebrenica, another Rwanda, which we are sadly ready to recognize written on that wall in front of us, unless something takes place.”

De Mistura said a maximum of 900 Nusra Front fighters would “need some guarantees” that they would be allowed safe passage into neighboring Idlib province. He also said that history would judge Russia and Syrian leaders harshly if they used the presence the extremists in eastern Aleppo as an “easy alibi” to raze the area with missiles.

Russia appeared to be at least interested in the idea, with the TASS news agency quoting presidential spokesman Mikhail Bogdanov as saying it was “high time” the U.N. envoy suggested a way to get the former Nusra fighters out of the city.

Russian Foreign Minister Sergey Lavrov, currently in France, said his staff was studying the proposal in detail.

De Mistura said Thursday that only an estimated 8,000 rebel fighters are holed up in the eastern parts of Aleppo amid a government offensive in the northern Syrian city -- and that no more than 900 insurgents there are from the al Qaeda affiliate in Syria.

The Russian-backed Syrian offensive on rebel-held parts of Aleppo has in part spurred the United States to suspend its cooperation with Russia in trying to achieve a cease-fire in Syria.

In a BBC interview last week, Russian Foreign Minister Sergey Lavrov said that 50 percent of opposition fighters in Aleppo were from Nusra Front, “as confirmed by the United Nations,” according to a transcript posted on Russian diplomatic websites.