BEIRUT: At least 14 people were killed when multiple explosive devices struck a police truck transporting prisoners in a tense area of northwestern Syria on Saturday, the country's state-run news agency and an opposition group said.

SANA news agency blamed the attack on "terrorists" and said it occurred on the Idlib-Ariha highway, an area near the Turkish border that 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 wounded, some of them in critical condition, it said.

The British-based opposition activist group, the Syrian Observatory for Human Rights, confirmed the incident Saturday and put the toll at 15 dead prisoners.

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.

The activist spoke on condition of anonymity for fear of reprisals.

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 wounded in the fighting, some of them in serious condition.

The 10-month uprising against Syrian President Bashar Assad has turned increasingly militarized and chaotic as more frustrated regime opponents and army defectors arm themselves and fight back against government forces.

The conflict in Syria has marked the most serious challenge to Assad, who took over from his father in 2000. The U.N. estimates some 5,400 have been killed since March, when the uprising began.

The capital has seen three suicide bombings since late December which the government blamed on terrorist extremists.

Security officials in Lebanon meanwhile said the Syrian navy arrested three Lebanese fishermen and confiscated their boat Saturday in Lebanese waters off the northern town of Arida.

The two brothers and their nephew were taken after Syria soldiers aboard a naval vessel fired in the direction of the boat, the officials said. They spoke on condition of anonymity in line with regulations.

After the incident, angry residents of Arida blocked the highway linking Lebanon and Syria for hours with burning tires.