A unit of about 80 FARC guerrillas presumably operating out of Venezuela killed 12 Colombian soldiers along the border, officials said.
"It's a setback, a setback in the war we are waging and in which we are making progress and are winning," President Juan Manuel Santos said.
The Colombian president said he spoke with his Venezuelan counterpart, Hugo Chavez, by telephone Monday morning and again in the afternoon to discuss the incursion by the rebels, who returned to Venezuela after staging the attack.
Chavez reiterated his position that the rebels' presence would not be tolerated in Venezuela and said he ordered the mobilization of military units in the border region, Santos said.
Revolutionary Armed Forces of Colombia, or FARC, guerrillas attacked the soldiers Monday in the northern province of La Guajira, which borders Venezuela's Zulia state.
"We are ratifying our position - we are not going to allow incursions by any armed force, whatever its nature, into Venezuelan territory," Chavez said Monday.
The Venezuelan president re-established relations with Colombia in August 2010, when Santos took office.
Former President Alvaro Uribe, who governed Colombia from 2002 to 2010, accused Chavez of tolerating the rebels' presence in Venezuela, a charge that led to the rupturing of relations in mid-2009 by Caracas.
The attack was staged by "a very large group (of rebels) who presumably came from Venezuela and who presumably fled to Venezuela," army commander Gen. Sergio Mantilla said in a press conference in Maicao, where the attack occurred.
About 80 to 90 FARC rebels armed with assault rifles and gas cylinders packed with explosives carried out the attack, Mantilla said.
An officer, a non-commissioned officer and 10 enlisted men were killed in the fighting, while four other soldiers were wounded.
Officials did not provide any rebel casualty figures.
Soldiers from a mechanized unit and a combat engineers unit were providing security for a civilian crew repairing a transmission tower that carries electricity to Venezuela.
The attack was a FARC "reaction" to the effective operations being carried out by the security forces against the rebels, Santos said.
Killings of guerrillas in combat by the security forces are up 39 percent and captures have soared 72 percent this year, the president said.
The Colombian government has made fighting the FARC a top priority and has obtained billions in U.S. aid for counterinsurgency operations.
The FARC is on both the U.S. and EU lists of terrorist groups. Drug trafficking, extortion and kidnapping-for-ransom are the FARC's main means of financing its operations.
The FARC has suffered a series of setbacks in recent years at the hands of the Colombian security forces.
Alfonso Cano, the FARC's top leader, was killed on Nov. 4 in a military and police operation that the government hailed as the biggest blow to the FARC in its nearly 50-year history.
Cano, a 63-year-old intellectual who had entered the ranks of the FARC 30 years ago, was killed in in a remote area of the southwestern province of Cauca a few hours after fleeing a bombardment.
The FARC also suffered a series of blows in 2008, with the biggest coming in July of that year, when the Colombian army rescued a group of high-profile rebel-held captives: former presidential candidate Ingrid Betancourt, U.S. military contractors Thomas Howes, Keith Stansell and Marc Gonsalves, and 11 other Colombian police officers and soldiers. EFE