People who were evacuated from the two villages of Kefraya and al-Foua walk near buses, after a stall in an agreement between rebels and Syria's army, at insurgent-held al-Rashideen, Aleppo province, Syria April 15, 2017. [Photo/Agencies]

The British-based Syrian Observatory for Human Rights said the delay was because rebels from Zabadani, one of the towns included in the deal, had not yet been granted safe passage out.

The agreement is one of several concluded in recent months that has seen President Bashar al-Assad's government take back control of areas long besieged by his forces and their allies.

In the latest deal, hundreds of rebels and their families evacuated the town of Madaya near Damascus, and were taken to the government-held city of Aleppo. From there they will travel to Idlib province, an insurgent stronghold.

In return, pro-government fighters and residents from the Shi'ite villages of al-Foua and Kefraya, both surrounded by Idlib rebels, have left the area and reached Aleppo's outskirts.

On Saturday, those from Madaya sat outside rows of coaches in a bus garage in government-held Aleppo, waiting to move onto Idlib, pictures sent by a pro-opposition activist showed.

Meanwhile residents from the Shi'ite villages were still waiting in insurgent territory on Aleppo's outskirts to cross into the city, the Observatory and a witness said.

The pro-opposition activist said insurgents were saying that a smaller number of pro-government fighters had left the Shi'ite villages than was agreed.

A Madaya resident, speaking from the bus garage in Aleppo, said people had been waiting there since late on Friday night, and were not being allowed to leave.