Chevron oil says it has cut production by 13,000 barrels a day in Nigeria due to a damaged pipeline.
The pipeline is in the oil-rich Niger Delta region, where militants have carried out a string of recent attacks on foreign oil company installations.
Local media reports say militants attacked a Chevron facility. Chevron said Wednesday that the cause of the damage is under investigation.
Most of the attacks in the Niger Delta have targeted facilities operated by Royal Dutch Shell. The attacks have forced Shell to slash its Nigerian oil production by 455,000 barrels a day, putting pressure on world oil prices.
Armed militants still hold nine foreign oil workers who were kidnapped in an attack on a Shell export terminal last month.
The militants, from the Movement for the Emancipation of the Niger Delta, are demanding greater local control over the region's oil wealth and the release of two jailed ethnic Ijaw leaders.
Four other oil workers kidnapped by militants in January were released after 19 days.
Some information for this report was provided by AFP and AP.