At least 14 Colombian marines were killed and about 25 others hurt when leftist rebels attacked a military post, the navy has said.
The attack is blamed on members of Colombia's biggest rebel group
Government forces in river gunboats, a helicopter and a gunship were pursuing the Farc guerrillas held responsible, the military said on Tuesday.
The pre-dawn raid with homemade rockets in Iscuande, south-west Colombia, was the deadliest rebel attack in months.
It comes amid a government campaign against rebels in southern Colombia.
The attack suggests the Farc rebels retain the capability of striking against the government despite its offensive, called Plan Patriot.
Adm Mauricio Soto, commander of Colombia's navy, said a lieutenant who commanded the jungle outpost and a dozen other marines had been killed.
"We deeply regret the death of our men, but this was a group of Colombians who died defending their country and its people," he said, according to the Associated Press news agency.
The rebels used large gas cylinders converted into rockets to attack the base, an army statement said.
The area where the Iscuande River empties into the Pacific Ocean in the southern province of Narino is known as a major cocaine-producing centre.
The navy said Colombian counter-narcotics troops were involved in the hunt for the rebels.
Among those attacked were campesino, or peasant, marines, natives of the area who are stationed at local military posts after a short period of training.
Their deployment forms a major part of President Alvaro Uribe's strategy to end a 40-year insurgency by members of the Farc (Revolutionary Armed Forces of Colombia) to an end.
The US has provided about $3bn (£1.6bn) in mostly military aid to Colombia since 2000 to combat both the rebels and the country's huge cocaine industry.