A roadside bomb and government shelling killed at least 19 people, most of them rebel fighters, in southern Syria's Daraa province on Thursday, a monitor said.
The Britain-based Syrian Observatory for Human Rights said an explosive device placed on a road by government forces detonated as a convoy of rebel fighters passed by.
"After the blast, civilians from nearby came to the scene and the regime shelled the area," said Observatory director Rami Abdel Rahman.
He said 13 of the dead were from local rebel factions, and at least three others were civilians.
The identities of the remaining fatalities had not been confirmed.
The monitor said the toll in the incident in the northwest of the province could rise further because several people were suffering serious injuries.
Syrian forces have been occasionally accused of planting explosives to target opposition fighters.
Daraa province is one of the last remaining bastions of rebel forces, who retain control of a majority of the region even as they have been routed elsewhere.
More than 320,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011.