Syrian rescue workers volunteering as first responders in rebel-controlled areas say at least 100 people were killed and 55 injured in a car bomb explosion that ripped through a makeshift bus depot in Aleppo province where evacuees were waiting to be transported to government-held areas.

The Syrian Civil Defense in Aleppo province, also known as the White Helmets, said its volunteers removed at least 100 bodies from the scene of the blast. The group said on Twitter that a car bomb caused the explosion.

Syrian state media put the death toll at 39, including civilians and rebel fighters. A rebel official said at least 30 of his fighters guarding evacuees died in the blast. ​President Bashar Assad's government blamed the attack on rebels.

The British-based Syrian Observatory for Human Rights put the death toll at 43, but said it was expected to rise.

​In footage aired on Syrian TV, bodies, including fighters, were seen lying alongside charred and destroyed buses. The channel said the car used in the bombing was carrying food aid but the rebels said the car had been parked in the area and abandoned.

The SANA news agency reported that the blast went off in the al-Rashideen area of Aleppo, where buses and ambulances were parked for more than 24 hours to transport 5,000 residents of the Shiite villages of Kefraya and al-Foua’a.

Residents from the two villages evacuated Friday, along with more than 2,000 from Madaya, an opposition-held town outside of Damascus besieged by government forces.

The contentious transfer of the population, hammered out after rebels lost control of Aleppo in December, stalled overnight as the government and rebels argued over who should be evacuated.

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 deal.

Syria state TV reported the transfer process resumed hours after the explosion, with dozens of wounded transported.

Read more:

Syria's civil war: Disturbing facts show cost of conflict
As the Syrian conflict turns 6 years old, conditions remain unbearable
U.S. launches cruise missile strike on Syria after chemical weapons attack

A senior rebel leader said 20 fighters guarding the buses were killed along with dozens of passengers. He spoke on the condition of anonymity because he was not authorized to speak to the media, according to the Associated Press. Yasser Abdellatif, a member of the powerful Ahrar al-Sham group that negotiated the transport deal, said at least 30 rebels were killed in the explosion.

Before the blast, Syrian Red Crescent teams distributed meals to the anxious evacuees.

“The people are restless and the situation is disastrous,” said Ahmed Afandar, a resident evacuated from the opposition area near Madaya. “All these thousands of people are stuck in less than (500 yards).” He said the area was walled off from all sides and there were no restrooms.

Afandar said people were not allowed to leave the buses for a while before they were let out. Food was distributed after several hours and by early afternoon the evacuees from rebel-held areas were “pressured” to sit back on their buses, Afandar said.

The evacuees from Madaya were expected to head to rebel-held Idlib, west of Aleppo.

Syrian state TV blamed the rebels for obstructing the transfer deal, causing thousands of evacuees to be stuck in bus depots overnight.

A resident of Zabadani — another rebel-held town to be evacuated — Amer Burhan saids no evacuation had even taken place from there.

The population transfer deal has been criticized by opponents as forced displacement and was not overseen by the United Nations.

Contributing: Associated Press