President Evo Morales has decreed he is nationalizing Bolivia's vast natural gas industry, sending soldiers to occupy gas fields and threatening to evict foreign companies unless they give the Andean nation control over the entire chain of production.

The move fulfills an election promise by the leftist president, who has forged close ties with Cuba's Fidel Castro and Venezuela' Hugo Chavez, to increase state control over Bolivia's natural resources, which he says have been "looted" by foreign companies.

Morales on Monday sent soldiers and engineers with Bolivia's state-owned oil company to installations and fields tapped by foreign companies -- including Britain's BG Group PLC and BP PLC, Brazil's Petroleo Brasileiro SA, Spanish-Argentine Repsol YPF SA, France's Total SA and Texas-based Exxon Mobil Corp.

The companies had six months to agree to new contracts or leave Bolivia, he said.

"The time has come, the awaited day, a historic day in which Bolivia retakes absolute control of our natural resources," Morales, Bolivia's first Indian president, said in a speech from the San Alberto field operated by Petrobras in association with Repsol and Total SA.

State television aired footage of soldiers and police standing guard outside some gas installations and petroleum company offices in the eastern city of Santa Cruz, where much of the industry is based.

Vice President Alvaro Garcia Linera said troops were sent to 56 locations nationwide.

"The looting by the foreign companies has ended," Morales declared.

Brazil is Bolivia's biggest natural gas client, followed by Argentina, and Brazil's demand has been rising rapidly due to power generation, cooking and automotive needs.

The president of Brazil's government-run oil company described the move Monday as an "unfriendly" act.

Petroleo Brasileiro SA, or Petrobras, is of the largest foreign investors in Bolivia and has spent $1.5 billion in that Andean nation since 1996.

"Evo Morales' decree was a unilateral measure adopted in an unfriendly way," Petrobras President Jose Sergio Gabrielli told the official Brazilian news service Agencia Brasil in Houston, Texas where he was taking part in an international oil conference.

"It obliges us to analyze very carefully our situation in the country."