Over 100 people were killed Saturday when a car bomb exploded near a bus carrying Syrian refugees, according to the Syrian Observatory for Human Rights.
The bombing targeted an area where people were gathering to leave their war-torn towns. An agreement between the Syrian government and the rebels allowed thousands of people to escape the daily violence.
The U.N. described the situation in those four towns as "catastrophic." All of them have been surrounded by the opposing army since 2015, and many people have died due to food and medicine shortages.
An attempt at similar evacuations failed in December after rebels burnt the vehicles that were going to be used to evacuate residents of the towns.
Both rebel and government forces were on the scene of the attack Saturday. Later in the day, the relocation of those residents continued.