At least 17 Colombian soldiers have been killed in fierce fighting with Marxist rebels, the military has said.
Colombian troops have been targeted twice already this month
The army said Farc rebels ambushed government troops in a remote jungle area in the country's north-west.
Another eight soldiers are missing, while 11 rebels have been killed, the army said. Earlier reports suggested 20 soldiers had died in the clash.
It is the deadliest attack since President Alvaro Uribe took power more than two years ago.
It follows the deaths of 23 troops last week in two other suspected Farc attacks in the south of the country.
President Uribe has pledged to end the 40-year insurgency by the Revolutionary Armed Forces of Colombia.
The president has frequently claimed that he has put the Farc on the backfoot by strengthening the armed forces and ordering tough tactics.
Simon Trinidad denies terrorism and drug-trafficking charges
But correspondents say these losses show that the rebels are still a powerful fighting force.
Meanwhile, a senior Farc commander, known as Simon Trinidad, has appeared in court in Washington for a pre-trial hearing after being extradited to the US in December.
The Farc leader, whose real name is Ricardo Palmera, pleaded not guilty to charges of terrorism and drug-trafficking.
The judge, Thomas Hogan, said the trial would not start for six months because complex legal paperwork was involved.