Colombia's FARC rebels killed seven soldiers and injured five in combat on Wednesday, the military said, in the worst blow against security forces since peace talks began late last year.
An unidentified number of the rebels were also killed in the fighting in southern Caqueta province, the military said in a statement.
Violence has intensified in recent weeks as the FARC, or Revolutionary Armed Forces of Colombia, steps up pressure on the government while the two sides seek a negotiated end to five decades of war in talks in Cuba.
The Marxist guerrillas, who lifted a unilateral ceasefire on Jan. 20, have increased attacks on civilian and military targets, taken hostages and blown up oil and energy infrastructure in a bid to force the government to suspend hostilities.
Colombian President Juan Manuela Santos has said the military offensive will end only if peace is reached.
The FARC, Latin America's biggest and oldest rebel group, plans to free two captive police patrolmen and a soldier as soon as Thursday. That move could ease some of the tension at the negotiating table caused by an escalation of the conflict.
While U.S.-backed strikes against the FARC have severely weakened the rebels and limited their ability to attack the country's economic drivers, the FARC is still able to put up a fight with the military.