The commander of the First Infantry Navy Brigade, Colonel Rafael Colon, said fighting began on Tuesday between government troops and leftist rebels of the Revolutionary Armed Forces of Colombia (FARC) and the National Liberation Army (ELN) in the Montes de Maria region.
He said at least 12 rebels and six soldiers had been killed in the clashes near the town of Carmen de Bolivar.
Colombia has been wracked by civil war for more than four decades.
FARC recently announced it would renew armed actions, following a pause in which it sought to negotiate with the government to release hostages in exchange for FARC rebels held in Colombian jails.
President Alvaro Uribe renewed his "get tough" approach on Wednesday when he approved the extradition of a top FARC commander, "Sonia", to the United States on drug charges.