The death toll from a bomb blast on a crowded Syrian bus convoy outside Aleppo has reached at least 112 people, the Syrian Observatory for Human Rights monitoring group said.

Syrian rescue workers the Civil Defence said they had carted away at least 100 bodies from the site of yesterday's blast, which hit buses carrying Shi'ite residents as they waited to cross from rebel into government territory in an evacuation deal between warring sides.

The British-based Observatory reported its new toll early on Sunday and said the number was expected to rise.

A damaged bus at the scene of the car bomb attack west of Aleppo. (AAP)

Buses at the evacuation point where an explosion hit at the Rashideen area, a rebel-controlled district outside Aleppo city. (AAP)

Those killed were mostly residents of the villages of al-Foua and Kefraya in Idlib province, but included rebel fighters guarding the convoy, the Observatory said.

There was no immediate claim of responsibility for the attack, which pro-Damascus media said was carried out by a suicide car bomber.

The convoy was carrying at least 5000 people including civilians and several hundred pro-government fighters, who were granted safe passage out of the two Shi'ite villages which are besieged by rebels.

Rescue and media personnel at the scene of a car bomb attack that targeted buses evacuating civilians west of Aleppo. (AAP)

Under the evacuation deal, more than 2000 people including rebel fighters were granted safe passage out of Madaya, a town near Damascus besieged by government forces and their allies.

That convoy was waiting at a bus garage in a government-held area on Aleppo's outskirts, a few kilometres from where the attack took place. Madaya evacuees said they heard the blast.

Meanwhile the evacuation of rebel fighters and their families from a neighbourhood in the central Syrian city of Homs has been delayed following the Aleppo attack.

The Observatory said the fifth phase of the rebel evacuation from al-Waer neighbourhood, the only district in Homs city that still has insurgents, was set to take place today.

The monitor expected the evacuation process in al-Waer to resume tomorrow.