Aid agencies have begun evacuating 500 wounded people from four besieged towns in Syria.
Half will be taken from Madaya and Zabadani, towns near the Lebanese border besieged by government forces.
The rest will be evacuated from Foah and Kefraya in north-west Syria, which are under siege by rebels.
In January, the UN warned hundreds of people trapped in Madaya were at risk of starvation if they were not allowed to flee immediately.
A spokesman for the UN secretary-general confirmed the plans are underway.
Stephane Dujarric told reporters on Wednesday the people are in "urgent need of life-saving medical attention".
Mr Dujarric added: "The sad thing is, we should not have to negotiate medical evacuations.
He gave no further details on the timing or where the people were being taken.
The move represents the largest evacuation since a fragile cease-fire made in February.
In December 2015, 460 people from four communities were evacuated.
Additional reporting by Associated Press
