"We will not let them in. We will fight with our lives, we will lead an armed struggle with traditional weapons if needs be," Alberto Pizango, an Achuar tribal leader, told a news conference.

The Achuars, in Lima to lobby Congress to annul the Block 64 concession, say their rivers, crops and land have been ruined by lead and mercury deposits left from past oil work.

Dozens of elderly Achuars have died this year from poisons that have leaked into ground water supplies, they say.

Burlington and Occidental won a contract with the Peruvian government in 1999 to exploit 1,976,843 acres (800,000 hectares) in the Loreto tropical rainforest region.

The two companies are now beginning oil exploration and promise investment in education and health centers in return for drilling on the land. No one from either company was immediately available to comment on the Achuar threat.

The Achuars say past work by Occidental and state-owned Petroperu means they are totally against oil exploration unless companies improve their environmental track record.

Occidental and Petroperu developed two blocks in the Peru-Ecuador border area during the 1970s and 1980s, selling the concession to Argentina's Pluspetrol in 1998.

Occidental said it spent $7 million cleaning up after it left but the Achuars say rusting oil pipes are still strewn across their lands and want the government to spent a promised $20 million on a clean-up.

Energy projects have become highly sensitive issues in Peru in recent months as the $1.6 billion Camisea gas project in southeastern Peru prepares to supply Lima next year.