Staffan de Mistura, United Nations Special Envoy for Syria, shows six with his hands as six days of the truce holding, during a news conference after a meeting of the Task Force for Humanitarian Access at the U.N. in Geneva, Switzerland, March 3, 2016. Denis Balibouse/Reuters The United Nations Special Envoy for Syria Staffan de Mistura offered on Thursday to go to eastern Aleppo and escort up to 1,000 Islamist fighters out of the city for the sake of a halt to the bombardment by Russian and Syrian forces.

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" for destroying the rebel-held besieged area, killing thousands of the 275,000 citizens, 100,000 of whom are children.

"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 told a news conference in Geneva.

"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," he said.

Syrian government forces seized around half of a key opposition-held neighborhood in Aleppo on Thursday in a new advance against rebels, the Syrian Observatory for Human Rights monitoring group said.

De Mistura said that there were a maximum of 8,000 fighters in eastern Aleppo, including those formerly known as Nusra Front, who have renamed themselves as Jabhat Fateh al Sham. At least 200 wounded civilians needed medical evacuation to save their lives.

The veteran diplomat addressed his plea directly to the former Nusra fighters: "And if you did decide to leave, in dignity with your weapons, to Idlib or anywhere you wanted to go, I personally am ready, physically ready, to accompany you."

Addressing himself to the Russians and the Syrian government, he asked: "Are you really ready to continue this type of level of fighting using that type of weapons, and de facto destroy the whole city of eastern Aleppo ... which is home to 275,000 people for the sake of eliminating 1,000 al Nusra fighters?"

The alternative was an immediate halt in the bombing in response to the Nusra fighters' departure, which would leave the local administration in place.

The U.N. should then be allowed to reach the population with humanitarian medical aid.