Three Lebanese shot and wounded by Syrian troops as they were crossing into northern Lebanon died of their injuries overnight, a medical official said on Wednesday.

A fourth person was wounded in the shooting but managed to get out of the car in which they were travelling when it was hit by gunfire near an unofficial border crossing at Raydani, the official said.

Late Tuesday a witness said Syrian troops opened fire at a car as it headed towards the Raydani crossing, wounding the passengers.

Three of them, members of the same family, were stuck bleeding in the car for more than 15 minutes before Lebanese army troops and Red Cross rescuers were able to reach them and take them to hospital where they eventually died.

The fourth wounded passenger was able to get out and drag himself to a nearby river that spans Lebanon and Syria, and was later found by Bedouins who live in nearby tents and rushed to hospital.

The latest deaths bring to six the number of people killed since October by Syrian troops who have regularly staged incursions into neighbouring Lebanon in recent weeks and opened fire on border villages.

Lebanon and Syria share a 330-kilometre (205-mile) border but have yet to agree on official demarcation.