The final stage of the evacuation of rebel-held districts of the Syrian city of Aleppo is reportedly facing delays.
Opposition activists said a convoy of buses was stuck overnight, forcing people to wait in freezing temperatures with little to eat or drink.
State media blamed rebels in nearby Idlib province, accusing them of preventing the simultaneous evacuation of two pro-government Shia towns there.
The Red Cross says some 25,000 people have left Aleppo in the past week.
They are being transported to rebel-held territory in the countryside west of Aleppo and in Idlib, where camps are being set up.
The Syrian Observatory for Human Rights, a UK-based monitoring group, reported on Wednesday that 60 buses – with room for about 3,000 people – were waiting to be allowed to leave eastern Aleppo.
This has forced civilians and rebel fighters wanting to be evacuated to stand outside in harsh winter conditions or take shelter inside nearby damaged buildings, according to activists and journalists on the ground.
The BBC’s James Longman in Beirut says it is unclear why the process has stalled.
But the Syrian Observatory said 21 buses were also waiting to evacuate sick and wounded people from the two towns in Idlib that are besieged by rebel forces.
The state news agency, Sana, said a convoy had been prevented from entering Foah and Kefraya, because of “differences among terrorist groups”. So far, 750 people have been evacuated from the towns.
A spokeswoman for the International Committee of the Red Cross, Ingy Sedky, told the BBC that being evacuated was tough for those involved.
“The evacuation is bringing some respite to the people, some relief, but still you can imagine the psychological and physical effect it has on them,” she said.
“Thousands of families are now leaving their homes, their belongings, leaving their memories behind, exploring other territories and starting all new.”
If the delays are resolved, our correspondent says, Wednesday could be the day the whole of Aleppo returns to government control.
The Syrian army seems determined to clear the area, he adds, and has been broadcasting announcements via loudspeaker, calling on the last of the rebels to leave before soldiers arrive.
But a spokesman for the Fastaqim rebel group, Ward Furati, told the Associated Press they “won’t leave until security of all the civilians has been fully guaranteed”.
Twenty UN observers are due to arrive soon to ensure civilians are protected.