Three soldiers were killed and another was wounded over the weekend when they triggered landmines planted by the FARC guerrilla group in a rural area outside Tibu, a city in northeastern Colombia, the army said.

The soldiers died Saturday in an area between the villages of Petrolea and Campo Dos, which are located in Norte de Santander province.

The soldiers were on a routine patrol when they entered the mine field in an oil-producing area, army 30th Brigade spokesmen said.

The landmines were planted by the 33rd Front of the Revolutionary Armed Forces of Colombia, or FARC, guerrilla group, the army said.

The soldiers died near the facility operated by Canada's Alange Energy, which was the target of a rebel attack earlier this month.

Landmines have been planted in 31 of Colombia's 32 provinces, the United Nations says.

Up to 100,000 of the weapons are estimated to have been planted around the Andean nation, the great majority of them by leftist rebels seeking to inflict casualties on soldiers and protect coca plantations that supply their extensive drug trafficking operations.

Almost all of the weapons are "non-industrial" homemade mines manufactured in guerrilla camps at low cost.

The FARC, Colombia's oldest and largest leftist guerrilla group, was founded in 1964, has an estimated 8,000 fighters and operates across a large swath of this Andean nation.

The Colombian government has made fighting the FARC a top priority and has obtained billions in U.S. aid for counterinsurgency operations.

The FARC, whose leader is Alfonso Cano, has suffered a series of setbacks in recent years.

The FARC's military chief, Jorge Briceño Suarez, known as "Mono Jojoy," was killed in an airstrike on Sept. 23, 2010.

On July 2, 2008, the Colombian army rescued former presidential candidate Ingrid Betancourt, U.S. military contractors Thomas Howes, Keith Stansell and Marc Gonsalves, and 11 other Colombian police officers and soldiers.

The FARC had been trying to trade the 15 captives, along with 25 other "exchangeables," for hundreds of jailed guerrillas.

The rebels' most valuable bargaining chip was Betancourt, a dual Colombian-French citizen the FARC seized in February 2002 whose plight became a cause celebre in Europe.

FARC founder Manuel Marulanda, who was known as "Sureshot," died on March 26, 2008.

Three weeks earlier, Colombian forces staged a cross-border raid into Ecuador, killing FARC second-in-command Raul Reyes and setting off a regional diplomatic crisis.

Ivan Rios, a high-level FARC commander, was killed that same month by one of his own men, who cut off the guerrilla leader's hand and presented it to army troops, along with identification documents, as proof that the rebel chief was dead.

A succession of governments have battled Colombia's leftist insurgent groups since the mid-1960s.

In 1999, then-President Andres Pastrana allowed the creation of a Switzerland-sized "neutral" zone in the jungles of southern Colombia for peace talks with the FARC.

After several years of fitful and ultimately fruitless negotiations, Pastrana ordered the armed forces to retake the region in early 2002. But while the arrangement lasted, the FARC enjoyed free rein within the zone.

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.