A rebel fighter stands as Syrians who arrived a day earlier from government-held villages of Fuaa and Kafraya wait in rebel-held Rashidin, west of Aleppo city, following delays in evacuating them as the hard-won evacuation deal ran into trouble on April 15, 2017. AFP/Getty Images

A bomb attack on Syrian bus convoy carrying evacuees to Aleppo has killed at least 43 people.

Pro-government media in Damascus said the blast was caused by a suicide attacker detonating a car bomb.

The blast hit the Rashidin area on Aleppo's outskirts that is controlled by opposition fighters.

State TV showed horrific images of bodies strewn on the ground and near buses. Humanitarian workers were among the victims of the attack, according to Arabic Asharq Al-Awsat.

The 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 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 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.

Belfast Telegraph