BEIRUT, Lebanon — Dozens of people were killed in the Syrian city of Aleppo on Saturday when a car bomb struck a group of buses carrying residents and fighters who had been evacuated from two besieged towns the day before.

The blast interrupted a deal between the government and the rebels to evacuate people from four villages that have suffered prolonged sieges.

Photographs from the scene posted on social media showed a line of buses destroyed in the blast, their windows blown out and charred bodies scattered in the road.

Syrian state television said at least 39 people were killed in what it called a terrorist attack. The Syrian Observatory for Human Rights, which opposes the government and tracks the war from Britain through contacts in Syria, said at least 43 people were killed.