A huge car bomb blast has killed at least 45 Syrians who were fleeing besieged government-held towns.

The explosion hit buses carrying families escaping settlements in the north of the war-torn country. State TV showed horrific images of bodies strewn on the ground.

The bomb was detonated at an evacuation point south of Aleppo city, where dozens of buses had been parked for more than 30 hours after a much-criticised population transfer deal stalled.

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.

A senior rebel official said 20 rebels who guarded the buses were killed as well as dozens of passengers.

Ahmed Afandar, a resident evacuated from his home town near Madaya, said dozens of buses carrying women, children and men were prevented from completing the journey toward rebel-held Idlib as planned.

Rami Abdurrahman, head of the Syrian Observatory for Human Rights, said the Syrian government and rebels who negotiated the deal 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 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.