The blast hit the buses, which were carrying Shi’ite Muslim evacuees, as they were lined up in the Rashidin district on the outskirts of the war-ravaged city.
They had been waiting to cross from rebel-held territory into the government-controlled city itself when the bomber struck.
The evacuees, alongside hundreds of pro-government fighters, had left two rebel-held villages in northwest Idlib province under an exchange deal which should have seen hundreds of Sunni insurgents and their families moved out of a government-besieged area near Damascus.
The immediate aftermath of the Aleppo bus convoy blast
But a delay in the agreement left all the evacuees stranded at two transit points on Aleppo's outskirts since late on Friday.
The deal was 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 British-based monitor the Syrian Observatory for Human Rights 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.
Thick black smoke rises from the Aleppo bus convoy blast scene
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.
Terrified women and children seek shelter after the Aleppo bus convoy blast
Get Quotes on Home Insurance
Several people were killed or injured in the blast
Countries that support and oppose the missile strikes in Syria
Mon, April 10, 2017
European countries have been broadly supportive so far as President Trump ordered missile strikes against a Syrian airfield from which a deadly chemical weapons attack was launched
1 of 7
SYRIA OPPOSES: President Bashar al-Assad's office denounced the U.S. strike as 'reckless, irresponsible behavior.'
A military media unit run by Damascus ally Hezbollah said a suicide attacker had detonated a car bomb near the convoy.
Pictures posted on state media showed what appeared to be the aftermath of the explosion, with bodies lying on the ground and fires belching out thick black plumes of smoke.
The buses were blackened by the blast with their windows blown out.