The Colombian military lost 13 members Saturday to an attack by FARC rebels, the country's army said in a statement. Two sergeants and 11 soldiers were killed.
Another soldier was injured in the attack and is receiving medical treatment, the military said.
This is the second incident in two months, reigniting tensions between the two sides which have been in peace talks that started late last year.
Last month, FARC admitted to capturing U.S. soldier Kevin Scott Sutay after he entered Colombia.
Colombian Defense Minister Juan Carlos Pinzon criticized the apparent kidnapping and called for his immediate release.
Pinzon said the apparent kidnapping showed the rebel group's promises have been disingenuous in the past.
"This organization had promised not to kidnap and they kidnapped an ordinary citizen," he said. "It is absurd that they continue abusing with promises that, it seems, continue to be false."
The leftist FARC, or Revolutionary Armed Forces of Colombia, has been at war with the government since the 1960s, making it the longest-running insurgency in Latin America.
There have been sporadic attempts at peace since the 1980s. The last fell apart in 2002. Then-President Andres Pastrana ceded an area the size of Switzerland to FARC, but ended negotiations after rebels launched a series of attacks across the country in an apparent bid to strengthen their position.