Founded in 1964 and inspired by the Cuban revolution, the ELN is the oldest surviving guerrilla group in the Americas, and controls key drug-producing regions of Colombia. Efforts to negotiate a peace settlement have repeatedly stalled.

The Thursday night attack on the rural military outpost in Aguachica, near the border with Venezuela, was the second deadly clash with security forces in a week, with at least eight soldiers killed.

"I categorically reject the ELN's terrorist action using drones and launching of explosive devices against a Military Base... with the regrettable loss of 6 of our soldiers and at least about 28 soldiers wounded," Defense Minister Pedro Sanchez wrote on social media early Friday.

In October, the United States imposed sanctions on Colombia's President Gustavo Petro for his alleged reluctance to target armed cocaine-trafficking cartels.

After taking power in 2022, Petro, himself a former guerrilla, attempted to engage well-armed cocaine-producing groups in talks, rather than conduct open warfare. But negotiations faltered.

Washington, conducting a campaign against alleged narco-trafficking off the coast of Venezuela, has warned Petro that he could "be next" over his country's mass cocaine production.

Colombia is the world's top cocaine producer, according to the UN.

The ELN, which is present in over a fifth of Colombia's 1,100-plus municipalities, vowed last week to fight for Colombia's "defense" in the face of US "threats of imperialist intervention."