A car bomb exploded near buses carrying Syrian evacuees, killing at least 39 people, according to Syrian state TV.
Dozens of buses were stopped outside Aleppo city, unable to proceed toward Idlib as part of a population transfer deal.
According to the deal, more than 2,000 residents, activists and gunmen from areas besieged by government forces were also evacuated. But as the government and rebels disagreed over the number of gunmen to be evacuated, the buses were left stuck at two separate parts, but adjacent parts of the city.
In footage aired on Syrian TV, bodies were strewn outside buses, including fighters. Some buses were charred and other gutted from the explosion as belongings hanged out of windows.