Armed fighters on Monday ambushed poll workers transporting ballots cast at an indigenous reservation in Colombia's regional elections, killing 12 security forces members who were protecting the group.
Authorities attributed the attack to the National Liberation Army (ELN), the country's second-biggest rebel group.
The fighters attacked the soldiers with explosives and shots in Guican municipality, in an area of the Andean highlands belonging to the U'wa indigenous group, Defense Minister Luis Carlos Villegas said.
Of those killed, 11 belonged to the army, and the other was a police officer. Six people remain missing, including two poll workers and an indigenous guide, Villegas said.
He said the group was transporting 130 ballots to the capital for tallying. They were cast on Sunday in gubernatorial and mayoral elections by residents of an U'wa Indian reservation on the edge of the popular Sierra Nevada del Cocuy National Park.
"They were safeguarding the political liberty of our U'wa brothers," he said in a press conference, adding that the military's top command had traveled to the area to oversee efforts to locate the missing as well as the attackers.
The ELN has yet to claim responsibility for the attack, and Villegas didn't provide details about guerrilla casualties or how the ambush unfolded.
The ELN has been engaged in exploratory talks with the government to draw up terms for a peace process but so far has refused to join the more powerful Revolutionary Armed Forces of Colombia in formal talks aimed at ending half a century of bloodshed. The preliminary negotiations with the ELN were kept under wraps for months and were revealed in June 2014.
The 2,000-strong ELN, founded by radical priests inspired by the Cuban revolution, has battled a dozen governments since it was founded in 1964 and has continued kidnapping and attacks on infrastructure even amid the exploratory talks.
"This shows that the ELN has not understood that this is the time for peace and not for war," President Juan Manuel Santos said.
"If the ELN thinks that these acts will win them political space or strengthen them in an eventual negotiation, they are completely wrong," he said, adding that he ordered the military to intensify its efforts against the rebels.
A nation of some 7,000 people, the U'wa are known for fiercely defending their ancestral homeland in the 1990s from drilling by Occidental Petroleum.
Bladimir Moreno, a tribal leader and the president of a group representing the U'wa, told The Associated Press that he plans to travel to the area to assist authorities with an investigation of the ambush. He said the isolated reservation, home to some 300 people, is a two-day trek from the nearest town and the community's sole telephone wasn't working in the immediate aftermath of the attack.