A bomb blast hit a bus convoy waiting to enter Aleppo on Saturday, killing and wounding dozens of people after an evacuation deal between Syria's warring sides halted and stranded thousands at two transit points on the city outskirts.

Pro-Damascus media outlets said a suicide attacker had detonated a car bomb and killed at least 22 people. Images posted by the outlets showed bodies lying next to charred buses with their windows blown out, and flaming vehicles belching out thick black smoke.

British-based monitor the Syrian Observatory for Human Rights counted 24 dead and dozens more wounded.

The blast hit buses in the Rashidin area on Aleppo's outskirts, which had been waiting to cross from rebel-held territory into the government-controlled city itself, carrying people evacuated from two Shi'ite villages on Friday.

The residents, alongside hundreds of pro-government fighters, had left the two rebel-besieged villages in northwest Idlib province under a deal where in exchange, hundreds of Sunni insurgents and their families moved out of a government-besieged area near Damascus.

But a delay in the agreement had left all those evacuated stuck at two transit points on Aleppo's outskirts since late on Friday.

Residents of al-Foua and Kefraya, the Shi'ite villages, were waiting in the Rashidin area.

The rebels and residents of Madaya near Damascus were waiting at the government-held Ramousah bus garage, a few miles away. They were to be transported to Idlib province, which the armed opposition controls.

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

The Observatory said the delay was caused by the fact that rebels from Zabadani, another town near Damascus included in the deal, had not yet been granted safe passage out.

A pro-opposition activist said insurgents blamed the delay partly on the fact that a smaller number of pro-government fighters had left the Shi'ite villages than was agreed.

Earlier on Saturday, at the transit point where the buses from al-Foua and Kefraya were waiting, one resident said he was not yet sure where he would live.

"After Aleppo I'll see what the rest of the group is doing, if there are any preparations. My house, land and belongings are all in al-Foua," Mehdi Tahhan said.

"WE HAD NO CHOICE"

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

"There's no drinking water or food. The bus garage is small so there's not much space to move around," Ahmed, 24, said.

"We're sad and angry about what has happened," he said. Many people felt that they had been forced to leave, he said.

"There was no other choice in the end - we were besieged inside a small area in Madaya."

Syria's opposition says the evacuation deals, which include areas of Aleppo and a district in the city of Homs, amount to forced displacement of Assad's opponents from Syria's main urban centres in the west of the country.

The agreements are also causing demographic changes, they say, because those who are displaced are mostly Sunni Muslims.

Syria's population is mostly Sunni. Assad is from the Alawite religious minority, often considered an offshoot of Shi'ite Islam.

He has been backed militarily by Russia, and by Shi'ite fighters from Iran and the Lebanese Hezbollah group in Syria's six-year-old conflict.

Assad has the military advantage over rebels in the west thanks to Russia's intervention in 2015, although the insurgents are still fighting back and have made gains in some areas.