Ten Colombian soldiers and an officer have been killed in an attack by rebels from the Revolutionary Armed Forces of Colombia (Farc), the army says.
The attack happened in the south-east of Colombia near the Venezuelan border. Two other soldiers were injured.
The BBC reports the troops were ambushed while patrolling an area about 400km from the capital Bogota, officials say.
Correspondents say it is the worst rebel attack in many months.
Last October, Farc rebels killed a total of 20 soldiers in two separate attacks.
In the latest attack, the rebels are believed to have launched home-made missiles at the soldiers.
The area where the attack took place, Arauquita, is under exploration by oil companies and is often targeted by rebels.
The soldiers killed on Saturday belonged to a company in charge of protecting oil installations and roads used by petrol tankers.