Aleppo, Syria (Reuters): A bomb blast hit a bus convoy waiting to cross into government-held Aleppo in Syria on Saturday, killing dozens of people evacuated from two Shi'ite villages the day before in a deal between warring sides.

The agreement had stalled, leaving thousands of people from both government-besieged and rebel-besieged areas stranded at two transit points on the city's outskirts, before the explosion occurred.

Late on Saturday buses began crossing into both government-held and rebel-held territory from the two transit points as the deal resumed, pro-Damascus media and the Syrian Observatory for Human Rights monitoring group reported.

But the incident underscored the difficulty carrying out any agreement between warring sides in a volatile and complex Syrian conflict which, in its seventh year, shows no signs of easing.

A media unit run by Damascus ally Hezbollah said the attack was carried out by a suicide car bomb and killed at least 40 people. The Observatory said more than 24 were killed and scores more wounded.

Footage on state TV showed bodies lying next to charred buses with their windows blown out, and vehicles in flames.

The blast hit buses in the Rashidin area on Aleppo's outskirts. The vehicles had been waiting since Friday to cross from rebel-held territory into the government-controlled city itself. Ambulances later took the wounded to hospital in Aleppo.

The convoy was carrying residents and pro-government fighters from the Shi'ite villages of al-Foua and Kefraya, which are besieged by rebels in nearby Idlib province, an insurgent stronghold.

They had left under a deal where, in exchange, hundreds of Sunni insurgents and their families were granted safe passage from Madaya, a government-besieged town near Damascus.

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

Residents of al-Foua and Kefraya waited in the Rashidin area.

Rebels and residents of Madaya meanwhile waited at the government-held Ramousah bus garage, a few miles away. They were to be transported to Idlib.

People waiting in the Ramousah garage heard the blast, and said they feared revenge attacks by pro-government forces. They circulated a statement on social media imploring "international organizations" to intervene so the situation did not escalate.

The evacuation deal 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 deals are unpopular with the Syrian opposition, who say they amount to forced displacement of Assad's opponents from Syria's main urban centers in the west of the country.

They are also causing demographic changes because those who are displaced are usually Sunni Muslims, like most of the opposition. Assad is from the minority Alawite sect and is supported by Shi'ite regional allies.

It was unclear who carried out Saturday's bombing attack.

The exact reasons for the delay in completing the evacuation deal were also unclear.