Six people were hurt when unknown assailants shot at two buses transporting employees of U.S.-based coal miner Drummond in the northern province of Cesar, the Colombian military said.

The attack took place on a stretch of road between the towns of Codazzi and San Diego, the commander of the army's 10th Armored Brigade, Eduardo Zapateiro, told La W radio.

Three passengers aboard the first bus suffered bullet wounds, while three people traveling on the second vehicle were hurt when the bus ran off the road.

The drivers of several other buses carrying Drummond workers were alerted and pulled over before reaching the site of the shooting.

Authorities did not immediately attribute the attack to any particular group, though well-armed criminal gangs and leftist guerrillas are known to operate in the area.

Police and army troops were deployed to the scene. EFE