The Syrian Observatory for Human Rights said the blast targeted buses carrying residents evacuated from the northern towns of Fuaa and Kafraya under a deal reached between the regime and rebels.
Bodies were still being recovered from the attack at a transit point in Rashidin, west of Aleppo, according to the Observatory.
"The suicide bomber was driving a van supposedly carrying aid supplies and detonated near the buses," the monitoring group said.
Thousands of evacuees had been stuck on the road because of a disagreement over the number of rebels allowed to leave two other towns included in the deal, but the process restarted following the blast, the Observatory said.
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AFP's reporter in rebel-held Rashidin saw several bodies, body parts and blood scattered on the ground.
The bombing took place as thousands of evacuees from Fuaa and Kafraya waited to continue their journey to regime- controlled Aleppo, the coastal province of Latakia, or Damascus.
Syria's war has left more than 320,000 people dead since erupting in 2011, with more than half the population forced from their homes and hundreds of thousands enduring siege-like conditions.
It has sucked in regional and international powers and allowed jihadist groups to seize vast areas of the country.
US-backed fighters reached the outskirts of a key jihadist-held town in northern Syria on Saturday as part of an offensive aimed at the IS bastion of Raqa.
The town and a vast nearby dam are considered key prizes in the broader offensive for Raqa, the de facto Syrian capital of IS's self-proclaimed "caliphate", about 55 kilometres to the east.
An SDF military source said Saturday that clashes were fierce and that the alliance's forces were "trying to penetrate the town from the east and west".
The alliance was reported to have advanced overnight after driving the jihadists from two areas just southeast and southwest of the town.
He said heavy clashes were under way around the two suburbs as IS attempted to counter-attack.
The SDF launched its campaign for Raqa in November and has since captured most of the surrounding province.
It has been backed by US-led coalition air strikes, along with advisers and even an American Marines artillery battery.
Raqa was home to around 240,000 residents before 2011 and more than 80,000 people have fled to the city from other parts of the country since the start of Syria's civil war.
Backed by Russia and local militias, pro-government forces have made a string of recent gains.
The government and rebels have brokered a series of deals to evacuate people from besieged areas, which Damascus touts as the best way to end the violence. Rebels say they are forced out by siege and bombardment.
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