Exxon Mobil Corp. (XOM.N) in Nigeria declared force majeure on oil liftings from different terminals in the country following industrial action by the company's in-house workers union, the company said on Monday in a statement.

Nigeria is trying to lift production to 1.6 million barrels per day (bpd), recovering its long time position as the largest African producer of the Organization of the Petroleum Exporting Countries (OPEC).

In the third quarter last year, production in Nigeria fell behind Angola to about 1 million bpd as companies including Shell PLC (SHEL.L) and TotalEnergies TTEF.PA seek to exit or scale back their presence, particularly in onshore operations which have been plagued by theft and spills for years.

Nigeria produced 1.38 million bpd in February, according to OPEC's latest report.

"We will continue to take all reasonable actions necessary to resolve the impasse as soon as possible," Exxon spokesperson Michelle Gray said in a statement on Monday.

Exxon has been trying to sell $1.2 billion in shallow-water assets in Nigeria, where it finds operations "challenging", the company told Reuters in February, while keeping deep-water assets further from the coast.

Oil prices turned lower on Monday morning as investors mulled over a possible May interest rate hike by the U.S. Federal Reserve, which could dampen economic recovery hopes. Brent crude futures were down 55 cents, or 0.6%, at $85.76 a barrel at 1240 GMT.