SANTA CRUZ DE LA SIERRA, Bolivia — Soldiers guarded natural gas fields and refineries after Bolivia's leftist president ordered the nationalization of the sector, threatening to evict foreign companies unless they give Bolivia control over production within six months.

President Evo Morales announcement Monday fulfils an election promise to increase state control over Bolivia's natural resources, which he says have been "looted'' by foreign companies. It also solidifies his role along with Venezuela's Hugo Chavez and Cuba's Fidel Castro in Latin America's new axis of leftist leaders opposed to U.S. and corporate influence in the region.

About 100 soldiers took control of the Palmasola refinery in the eastern city of Santa Cruz, some carrying assault rifles, while others carried anti-riot gear. Most stood in front of the gates of the refinery, which is run by Brazil's Petroleo Brasileiro SA, or Petrobras.

"Our mission is to guarantee the normal operations'' of the facility, said unit commander Capt. Jorge Lenz.

Soldiers and engineers with Bolivia's state-owned oil company were ordered to installations and fields tapped by foreign companies -- including Britain's BG Group PLC and BP PLC, Petrobras, Spanish-Argentine Repsol YPF SA, France's Total SA and U.S.-based Exxon Mobil Corp.

The companies have six months to agree to new contracts or leave Bolivia.

"The looting by the foreign companies has ended,'' Morales, Bolivia's first Indian president, said in a speech from the San Alberto field in Santa Cruz operated by Petrobras in association with Repsol and Total SA.

Vice President Alvaro Garcia Linera said troops were sent to 56 locations around the country.

The announcement follows a trend by oil- and gas-rich Latin American countries to exact a larger share of profits from extraction of the fossil fuels.

It comes as Ecuador argues with Washington over a new oil royalties law and less than a month after Chavez ordered the seizure of oil fields from Total and Italy's Eni SpA when the companies failed to comply with a government demand that operations be turned over to Venezuela's state oil company, Petroleos de Venezuela SA.

Bolivia has South America's second-largest natural gas reserves after Venezuela. All foreign companies must turn over most production control to Bolivia's cash-strapped state-owned oil company, Yacimientos Petroliferos Fiscales Bolivianos, Morales said.

Multinational companies that produced 28 million cubic metres of natural gas daily last year in Bolivia will be able to retain only 18 per cent of their production, with the rest being given to YPFB, he said. Morales did not name the companies.

"We are monitoring the situation very closely,'' said Bob Davis, a spokesman for the world's largest oil company Exxon Mobil Corp., which has a 30 per cent interest in a non-producing field called Itau, which is operated by Total.

In Madrid, Spain's government expressed "deep concern'' about the decree to nationalize the hydrocarbons sector.

"The government hopes that in the 180 days period announced by the Bolivian president for foreign companies to regularize their current contracts, there is authentic negotiation and dialogue between the government and the different companies in which each other's interests are respected,'' the Foreign Ministry said in a statement.

Petrobras called the Morales' decree an "unfriendly'' action that took the company by surprise.

"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 where he was taking part in an international oil conference. "It obliges us to analyze very carefully our situation in the country.''

Morales said the government would begin negotiations immediately with the companies to make sure they are willing to comply, but said they could be stripped of their privilege to operate in Bolivia if they don't sign new contracts within six months.