Around 50,000 people, the majority civilians, were still trapped in eastern Aleppo even as an evacuation deal was under way, the UN peace envoy for Syria and French foreign minister said.
"There are 50,000 people, including 40,000 civilians unfortunate enough to live in that part of the city. The rest are fighters, numbering between 1,500 and 5,000, and their families," the UN's Staffan de Mistura told reporters in Paris, alongside France's Jean-Marc Ayrault.
Turkey had earlier said 80,000 to 100,000 civilians were likely still trapped in eastern Aleppo.
"Our priority is for our UN colleagues to be present with the people (who have been evacuated) and that the fighters be respected under the terms of this deal," De Mistura said.
Under the terms of the evacuation agreement negotiated by regime backer Russia and rebel supporter Turkey, the people currently being evacuated from eastern Aleppo will be taken to the northwestern province of Idlib, an opposition bastion.
"We don't know what will happen in Idlib. If there is no political agreement and a ceasefire, Idlib will become the next Aleppo," De Mistura said.
Ayrault meanwhile repeated France's call for "the deployment, as quickly as possible, of UN observers, of all UN personnel who are already on the ground, and who can be deployed in the coming hours."
A UN Security Council meeting requested by France, and which is expected to take place on Friday, "will specifically examine the deployment of observers in order to ensure there are no abuses, no acts of revenge and that the civilian population is protected."
He added: "This can be done extremely quickly."
Ayrault, meanwhile, urged "a ceasefire for the whole country and for a return to negotiations."
Hundreds of people were evacuated from eastern Aleppo on Thursday, after a devastating month-long regime offensive carried out with the backing of Russian war planes.