BOGOTA, May 3 (Reuters) - Guerrilla land mines killed six Colombian soldiers on Saturday as the army pushed into rebel territory to track down key leaders of the country's biggest insurgent group, military sources said.

The incident, in the northeast province of North Santander, took place close to where the left-wing rebels bombed a major oil pipeline on Tuesday.

The insurgents are fighting a government bid to retake areas long held by the Revolutionary Armed Forces of Colombia, or FARC. Two members of the FARC's seven-man secretariat were killed earlier this year as part of President Alvaro Uribe's U.S.-backed crackdown on the rebels.

Thousands are displaced, maimed by land mines or killed every year in this decades-old war involving the FARC and other groups funded by Colombia's multibillion-dollar cocaine trade.

Tuesday's pipeline bombing happened just hours after U.S. Ambassador William Brownfield visited the area to show how security has improved due in part to billions of dollars in U.S. aid. (Reporting by Hugh Bronstein; Editing by Xavier Briand)

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