Marxist FARC rebels attacked a Colombian army unit and killed 11 soldiers, the army said.
The troops were hit by an "indiscriminate" strike on Saturday by rebels with the 10th front of the Revolutionary Armed Forces of Colombia (FARC) in Arauca department, in the east of Colombia, the army said.
A mid-ranking official and 10 privates were killed, the statement said. Two other people were injured, it added.
The FARC officially renounced abduction of civilians late last month, but still holds 10 members of Colombian security forces, though it has pledged to release them soon.
Colombian President Juan Manuel Santos says he will open a direct dialogue with the FARC only when all hostages are released and the group vows to cease "terrorist" actions. He also wants the FARC to stop recruiting children.
The FARC, believed to have 8,000 members, has been at war with the government since 1964. It began a campaign of kidnappings in the mid-1980s, seizing army hostages to serve as bargaining chips for FARC prisoners.