Clashes between rebels and the army in Colombia's main coca growing region left 30 soldiers and 26 guerrillas dead Friday, the military said.
They were the heaviest casualties since a U.S.-backed anti-narcotics offensive got underway late last year.
The clashes broke out at an army base near Puerto Leguizamo in the southern state of Putumayo, a launching point for Colombian marine operations against rebels and drug traffickers.
The army said it sent reinforcements and was pursuing retreating guerrillas of the Revolutionary Armed Forces of Colombia, or FARC.
Only six slain guerrillas were recovered, but army Gen. Nestor Ramirez put the number of dead at 26.
Also Friday, police arrested 57 alleged members of a Colombian gang accused of seizing oil workers in Ecuador last year and killing at least one American hostage, Ronald Clay Sander, 54, of Missouri.
Police said the group was believed responsible for at least eight kidnappings in Ecuador since 1990, abducting at least 12 Americans.