At least 100 people have been killed in the blast near evacuation buses outside Aleppo city, the Syrian opposition rescue service says.
At least 22 people have been killed in an explosion near evacuation buses in northern Syria, Syria TV says.
A war monitor has put the death toll at 24 in the area controlled by opposition fighters.
Today's explosion hit an evacuation point south of Aleppo city, where dozens of buses have been parked for more than 30 hours as a much-criticised population transfer deal stalled.
The state TV channel said the explosion was caused by a car bomb.
State TV showed horrific images of bodies strewn on the ground and near buses.
The Britain-based Syrian Observatory for Human Rights said at least 24 were killed.
A senior rebel official said 20 rebels who guarded the buses were killed as well as dozens of passengers.
The government and rebels disagreed over the number of gunmen to be evacuated, leaving the buses stuck.
The blast happened after thousands of Syrians being evacuated from their besieged towns had spent the night on buses at an exchange point.
Ahmed Afandar, a resident evacuated from his home town near Madaya, said dozens of buses carrying women, children and men were not being allowed to proceed toward rebel-held Idlib as planned.
"The people are restless and the situation is disastrous," said Mr Afandar. "All these thousands of people are stuck in less than half a kilometre (500 yards)."
He said the area was walled off from all sides and there were no toilets.
Rami Abdurrahman, head of the Syrian Observatory for Human Rights, said the Syrian government and rebels who negotiated the deal have differed over the evacuation of gunmen from the towns.
Dozens of buses carrying people from pro-government areas Foua and Kfraya, and from opposition towns Madaya and Zabadani, were parked at separate parts of the edge of Aleppo city, separated by walls, Syria TV showed.
An opposition representative, Ali Diab, told the pan-Arab Al-Arabiya TV that fewer armed men than agreed to were evacuated from the pro-government areas, violating the terms of the agreement.
Meanwhile, an Iraqi military statement said the Syrian air force has carried out a series of air strikes against so-called 'Islamic State' (IS) militants inside Syria, with one killing a leading member of the group in Raqqa, the IS de facto capital.
The statement said the strikes were against IS positions in Raqqa and Deir el-Zour, and near the Iraqi border. The statement described the targets as "the biggest positions for senior terrorists".
The statement said Abu Bakir al-Habeeb al-Hakim was killed in one of the strikes, describing him as a leading member of IS in Raqqa.
BREAKING: Syria TV: Population transfer resumes after blast near evacuation buses outside Aleppo city killed dozens.— The Associated Press (@AP) April 15, 2017