An attack involving explosives dropped from drones, followed by a clash inside a military compound in north-eastern Colombia, has killed at least three soldiers and wounded four, the Colombian army said on Saturday.

The attack took place late on Friday and targeted a military unit in the municipality of Ocana, in the Norte de Santander department, which borders Venezuela. Two soldiers died at the site and a third after receiving medical attention, the military said.

Local authorities blamed the attack on the rebel National Liberation Army (ELN).

Colombia's president, Abelardo de la Espriella, condemned the attack. "For every police officer or soldier murdered, we will bring the full weight and legitimate force of the state down upon the criminals," he said.

The ELN is an armed group active since 1964. Much of its forces are based in the Catatumbo region, which includes parts of the Norte de Santander and Cesar departments. The group also harbours dissident factions of the now-defunct Revolutionary Armed Forces of Colombia (FARC) that did not join a peace agreement a decade ago.

Both groups are vying for control of the strategic region, which connects to the porous border with Venezuela and is one of Colombia's primary coca-producing areas.

The attack is the latest in a series of incidents in the region. Earlier this week, at least one civilian was killed and 11 others were injured after a car bomb exploded outside a police station in north-eastern Colombia.

In August, an explosion outside a police station in Cucuta injured 11 police officers and killed a police dog. Officials also attributed that attack to the ELN.