Colombia's second-largest rebel group, the ELN, killed 12 members of the security forces in the central province of Boyaca on Monday, the government said, as the troops transported votes from the country's regional elections.
The National Liberation Army, or ELN, attacked the soldiers with explosives and shots in mountainous Guican municipality, in an area belonging to the Uwa indigenous group, Defense Minister Luis Carlos Villegas told reporters.
The 11 soldiers and one police officer who were killed were transporting votes from Sunday's gubernatorial and mayoral elections.
Three soldiers were injured. Two soldiers, a police patrolman, two election officials and one member of the Uwa tribe remain missing, the defense ministry said in a statement.
The 2,000-strong ELN is smaller than the country's more famous rebel group, the Revolutionary Armed Forces of Colombia, or FARC, which has been in peace talks with the government for 3 years.
The ELN, founded by radical priests inspired by the Cuban revolution, has been engaged in closed-door talks with the government to draw up terms for a formal peace process. The preliminary negotiations were kept under wraps for months and first revealed in June 2014.
"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 think that these acts will win them political space or strengthen them in an eventual negotiation, they are completely wrong," he said, adding that he had ordered the military to intensify its efforts against the rebels.
The group has battled a dozen governments since it was founded in 1964 and has continued kidnapping and attacks on infrastructure even amid the exploratory talks.
The ELN has sought peace before, holding negotiations in Cuba and Venezuela between 2002 and 2007. Experts say there was a lack of will on both sides to agree a final peace plan.
Colombia's five decades of conflict between leftist rebels, right-wing paramilitaries and government troops has killed more than 220,000 people and displaced millions.