LA PAZ, April 16 (Reuters) - Bolivia put natural gas fields and pipelines under military control on Monday in the southeast province of Chaco to stave off potential protests that could affect exports to Brazil and Argentina.

Local politicians and civic leaders in Gran Chaco, which belongs to the Tarija department where 80 percent of Bolivia's natural gas reserves lie, had threatened to occupy fields or take over pipeline control stations to cut exports starting on Tuesday.

The protest planned to draw attention to Gran Chaco's claim to rights over the large Margarita natural gas field, which holds about one third of the country's gas reserves.

"Bolivia has international agreements that must be protected and guaranteed," said Gen. Freddy Bersatti, head of the army, announcing the military action.

Spain's Repsol-YPF <REP.MC>, Brazil's Petrobras <PETR4.SA> and France's Total <TOTF.PA> operate different natural gas fields in the area.

Bolivia nationalized its energy industry a year ago, forcing foreign operators to pay much higher taxes and making the government the owner of energy reserves.

Gen. Wilfredo Vargas, commander in chief of the armed forces, said the military was protecting the operations, not controlling them.

Protests began five days ago in Gran Chaco with hunger strikes by lawmakers and city officials, and on Monday took more force with blocks on highways, including the main border crossing to Argentina.

Gran Chaco and neighboring O'Connor province both lay claim to the Margarita field, the biggest reserve in the country, with an estimated 15 billion cubic feet of natural gas. Margarita is still not exploited on a large scale.

Tarija Gov. Mario Cossio is set to decide by the end of this month which province will have the right to 45 percent of the approximately $100 million in annual taxes that could come from Margarita natural gas sales to Argentina in the future.

The government expects Bolivia's 30 million cubic meters a day of natural gas exports to Argentina and Brazil to reach an annual value of $2 billion this year.

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