At least 12 Colombian soldiers have been killed in an attack by members of the Revolutionary Armed Forces of Colombia (FARC), the country's largest rebel group.
Four other troops were also wounded after the rebels ambushed them in a rural area in the coastal La Guajira province near the Venezuelan border.
"Unfortunately we have received reports of 12 soldiers dead and four wounded," said Yusti Maria Lopez, a security official in La Guajira.
Eliecer Suarez, a regional army commander, said the rebels attacked his troops in the Serrania de Perija mountain range. He also said that the fighters also retreated back into Venezuela after the ambush.
The FARC has long used Venezuelan territory for sanctuary, rest and medical treatment.
Eurpides Pulido, La Guajira's local mayor condemned the attack, which rocked a quiet and isolated area in which numerous indigenous tribes live.
Hugo Chavez, Venezuela's president, said that he boosted border security following Monday's raid.
"We have stepped up air patrols as of this morning because we maintain our position, that we will not permit incursions of any armed force, whatever its type might be, into Venezuelan territory," Chavez said on state TV.
The rebel group has stepped up attacks on Colombian security forces over the past year in remote parts of the country.
In April, 15 soldiers were killed in fighting in the southern department of Caqueta, while in March 11 other soldiers were ambushed and killed in the eastern department of Arauca, which also borders Venezuela.