Bolivia's President Evo Morales speaks in Tarija, Bolivia

Bolivian President Evo Morales has signed a decree to nationalize the country's energy industry and ordered troops to immediately occupy natural gas fields to ensure production.

President Morales issued the decree in southern Bolivia Monday at the San Alberto gas field, which is operated by Brazil's state-owned Petrobras company. Mr. Morales said this was a historic day in which Bolivia retakes absolute control of its natural resources.

The president said foreign energy companies will have to sign new operating contracts within 180 days or leave the country and turn over most production control to Bolivia's state-owned oil company, known as YPFB.

Monday's decree is expected to affect foreign companies such as Spain's Repsol, French group Total, the U.S. company ExxonMobil, British Petroleum as well as Petrobras.

Officials from Petrobras could not be reached for comment as Monday was a holiday in Brazil. In the United States, an ExxonMobil spokesman Bob Davis said the company is monitoring the situation.

Bolivia has the second-highest natural gas reserves in South America after Venezuela.