Staffan de Mistura offers to personally escort rebel fighters out of Aleppo to help save 275,000 trapped civilians.
The UN’s Syria envoy on Thursday made an impassioned appeal to save eastern Aleppo, warning the city faced total destruction and urging Jabhat Fateh al-Sham fighters to leave the rebel-held east so civilians can get aid.
"In a maximum of 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 east of Aleppo has been hammered by a Russia-backed government offensive, including multiple air attacks on hospitals, with hundreds of civilians killed and thousands wounded.
De Mistura noted the presence of Jabhat Fateh al-Sham in the city has been used as a justification by Moscow and Damascus for the continued assault.
Formerly known as al-Nusra Front, the group recently changed its name following a break with al-Qaeda, but many still see the two groups as tied.
"Can you please look at my eyes," de Mistura said in a direct appeal to the group’s leaders, before pleading with them to quit Aleppo.
"If you decide to leave with dignity ... I am personally ready to physically accompany you," the UN envoy pledged.
He said history would judge Syria and Russia if they used the presence of about 900 Fateh al-Sham 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.
Aid deliveries to the besieged rebel-held neighbourhoods of Aleppo have been all but impossible since government forces seized the last supply route in July.
President Bashar al-Assad said on Thursday that rebels holed up in Aleppo can leave with their families if they lay down their arms, vowing to press on with the assault on Syria’s largest city and recapture full control of the country.
However, rebels said they had no plan to evacuate Aleppo, the last major urban area they control, and denounced the amnesty offer as a deception.
"It’s impossible for the rebel groups to leave Aleppo because this would be a trick by the regime," Zakaria Malahifji, a Turkey-based official for the Fastaqim group which is present in Aleppo, told Reuters.
"Aleppo is not like other areas, it’s not possible for them to surrender."
The government also sent text messages to the mobile phones of some of those people trapped in the besieged sector, telling them to repudiate fighters in their midst.
Speaking to Danish television, Assad said he would "continue the fight with the rebels till they leave Aleppo. They have to. There’s no other option."
On the ground, Syrian government forces seized about half of a key opposition-held neighbourhood in Aleppo on Thursday in a new advance against rebels, the Syrian Observatory for Human Rights monitor said.
The British-based group said fighting had been raging in the Bustan al-Basha quarter near the city centre.
"We are in an emergency mode regarding Syria, Aleppo, and the future of the this conflict," de Mistura said in Geneva.
The UN envoy described the suspension earlier this week of Syria ceasefire talks between Russia and the US as a "serious setback".
French Foreign Minister Jean-Marc Ayrault was expected to push on Thursday for a UN Security Council resolution to bring about a new Syria truce.
Ambassadors of the Security Council were to meet at the French Mission to the UN later on Thursday to discuss the proposed resolution, which "calls for an immediate ceasefire in Aleppo and the grounding of all aircraft over Aleppo", according to Al Jazeera’s James Bays, reporting from the UN headquarters in New York.
Government troops, with Russian air support and Iranian and militia ground forces, launched an assault to retake all of Aleppo last month after a week-old ceasefire agreed to by Washington and Moscow steadily fell apart.
"The siege has been in place for weeks now. Food and fuel supplies are dwindling … bakeries are closing, there is no running water. According to Oxfam, 1.5 million people on both sides of the city do not have clean running water," Al Jazeera’s Zeina Khodr, reporting from Gaziantep along the Syria-Turkey border, said.
But the government and its allies are still pushing ahead with their offensive, retaking territory in the north of rebel-held Aleppo in recent days as opposition fighters stopped a second advance from the south.
The government is demanding that civilians leave the area, "but the only option it has given them is to leave to the western, government-controlled side of Aleppo, and people are afraid".
As recently as Wednesday night, Jabhat Fateh al-Sham published footage showing fighters training and preparing for battle in Aleppo.
"We still haven’t had a chance to speak to the people of Aleppo and to see whether they accept this ... because yes, at the end of the day, the people of Aleppo do not accept and share the ideology of Nusra. But they do feel the Jabhat Fateh al-Sham is the only one really standing up against the government, and it is the strongest force in the ground repelling the government [advance]," Khodr said.
The 900 or so Fateh al-Sham fighters are among the estimated 8,000 rebel fighters left in east Aleppo.