The attack took place as thousands of evacuees from the towns of Foua and Kefraya waited to enter Aleppo.
A suicide car bomb has hit a convoy of buses carrying civilians out of two besieged Syrian towns, killing at least 100 people and wounding several others.
The explosion took place at a transit point Rashideen area as thousands of evacuees from the regime-held, predominantly Shia towns of Foua and Kefraya waited on Saturday to continue their journey to regime-controlled Aleppo, the coastal province of Latakia, or Damascus, according to the Syrian Observatory for Human Rights.
"The suicide bomber was driving a van supposedly carrying aid supplies and detonated near the buses," the Observatory said.
It warned that the death toll was likely to rise given the "several dozen wounded" at the blast site.
More than 5,000 people who had lived under crippling siege for more than two years left the two towns, along with 2,200 evacuated from opposition-held Madaya and Zabadani, on Friday.
Thousands of evacuees from Madaya and Zabadani were also stuck in regime-controlled Ramusa, south of Aleppo.
The evacuation, brokered by regime ally Iran and rebel backer Qatar, is set to see more than 30,000 people evacuated in two stages.
The deal to evacuate the towns was the latest in a string of such agreements, touted by the rebels as the best way to end the fighting. The opposition says they have been forced out by the siege and bombardment.
More than 400,000 people have been killed and millions more displaced since the Syrian conflict erupted with protests against Syrian regime leader Bashar al Assad in March 2011.