BOGOTA: The Colombian army killed at least 18 suspected leftist FARC guerrillas in a battle Sunday near the Ecuadoran border, Defense Minister Rodrigo Rivera said.

"The operation is underway and the toll that I can give you so far is 18 losses among the terrorists of the FARC," he said, using the Spanish acronym for the leftist Revolutionary Armed Forces of Colombia.

Rivero gave no figures for army casualties in the fighting, which erupted early Sunday in area of southern Putumayo province where eight police officers were killed on September 10.

The guerrilla movement appears to have gone on the offensive, with 40 members of the security forces killed since the start of the month in a string of attacks and ambushes.

Two soldiers were killed Saturday in a clash with the FARC in the southwestern town of Tulua, the military said in a statement.

Rivera said the FARC is trying to weaken President Juan Manuel Santos, a former defense minister who succeeded Alvaro Uribe as president August 7.

The FARC, founded in 1964, have about 8,000 fighters, according to official estimates.

Another leftist group also aligned against the government, the National Liberation Army (ELN), is thought to have about 2,500 fighters.

BDST: 0828 HRS, September 20, 2010