At least 22 people were killed when a car bomb hit buses carrying Syrians being evacuated from towns besieged by rebels.
State media reported the April 15 blast was set off by a suicide bomber and at least 22 people were killed.
The British-based Syrian Observatory for Human Rights put the death toll at more than 24.
State media published photos of what appeared to be the aftermath of the explosion, showing bodies lying on the ground and fires belching plumes of smoke.
The buses were carrying thousands of Shi'ites who support the central government of Bashar al-Assad.
Тhey were being evacuated to Aleppo from villages along the frontlines of fighting between government forces and rebel groups.
Syria's population is mostly Sunni. However, Assad and much of the ruling elite is of the Alawite religious minority, often considered an offshoot of Shi'ite Islam.