A massive car bomb killed 18 people in the village of Kafat in Syria's central Hama province on Thursday, the Syrian Observatory for Human Rights NGO said.
The monitoring group said women and children were among the dead in the blast, which also killed members of a pro-regime militia.
The huge explosion, which was heard in the nearby city of Hama, took place near a village school, the Observatory added.
Syrian state television also reported the attack in Kafat, which is under government control.
It said 16 people had been killed in a "terrorist explosion" in the village, adding that there were tens of injuries.
The Observatory said the death toll could rise because many of those wounded in the blast were in critical condition.
Those killed came from different sects and included Sunni Muslims, as well as Ismailis and Alawites, who follow different offshoots of Shiite Islam.
President Bashar al-Assad and many of his inner circle are from the Alawite community.