The soldiers were traveling in a vehicle close to the border with Venezuela when they hit an "improvised explosive device," the army said in a statement.

In a tweet, army general Eduardo Zapateiro described the attack as a "terrorist act that flagrantly violates international humanitarian law."

The attack is believed to have been carried out by National Liberation Army (ELN) guerrillas, although the group has not commented on it.

Colombia's Defense Minister Diego Molano vowed to take action against the attackers.

"We won't rest until we've neutralized them," he wrote on Twitter.

The ELN is the last active rebel group operating in Colombia and has been since the historic 2016 peace deal that ended a half century of armed combat by the significantly larger Revolutionary Armed Forces of Colombia (FARC) group.

Colombia's military high command accuses Venezuela's government of providing a safe haven for ELN guerrillas who then cross the border to carry out attacks.

Authorities in Bogota say there are around 900 ELN guerrillas hiding on the other side of the border, a claim denied by Venezuela President Nicolas Maduro.

The ELN began peace talks in 2017 with Colombia's then-president Juan Manuel Santos.

But his successor Ivan Duque called those off after a car bomb attack on a police academy in January 2019 killed 22 people.

The ELN has around 2,300 fighters and an extensive support network in urban centers.

Colombia is facing its worst wave of violence since the signing of the 2016 deal, according to the peace tribunal set up to investigate alleged rights abuses during the decades-long conflict.

Dissident FARC rebels, ELN guerrillas, armed drug-trafficking groups and rightwing paramilitaries are all involved in a bloody battle to control the lucrative cocaine and illegal mining markets.