At least 14 people died when a string of explosions struck a police truck transporting prisoners in a tense area of north-western Syria, the country's state-run news agency and an opposition group said.
Troops fought intense battles against defectors elsewhere in northern Syria, activists said, leaving "dozens" of people injured.
The 10-month uprising against Syrian president Bashar Assad has become increasingly militarised and chaotic as more frustrated regime opponents and army defectors arm themselves and fight back against government forces.
The Sana news agency blamed the attack on the police truck on "terrorists" and said it took place on the Idlib-Ariha highway, an area near the Turkish border which has witnessed intense fighting with army defectors recently.
Four bombs that went off in "two phases" hit the truck, and then attackers targeted an ambulance that arrived to assist the wounded, Sana reported. Six policemen who were accompanying the prisoners were also injured, leaving some in a critical condition, it said.
The British-based opposition activist group, the Syrian Observatory for Human Rights, confirmed the incident and claimed 15 prisoners were killed.
Rami Abdul-Rahman, director of the group, said the truck was hit by several roadside bombs, but it was not clear who was behind the attack. There was no immediate claim of responsibility, but members of the so-called Free Syrian Army are known to be active in the area.
A Syria-based activist said the area has several army encampments and is full of roadside bombs planted to target army tanks passing by, adding that the truck carrying prisoners may not have been the intended target.
Mr Abdul-Rahman and other activists in the country's northern Idlib province also reported heavy clashes between Syrian troops and defectors in the Jabal al-Zawiya region, along the Turkish border.
He said "dozens" of people from both sides were injured in the fighting, some of them seriously.