The U.S. oil company Chevron says it will suspend export contracts for much of its Nigerian output after an attack a key pipeline last week.

In a statement Thursday, Chevron said it will invoke the "force majeure" clause, excusing the company from its contracts until December 31.

Force majeure is a legal term, relating to events beyond a company's control that prevent it from meeting contractual obligations.

Last Friday, militants in southern Nigeria attacked a Chevron pipeline that feeds into the Escravos terminal. The company says the attack reduced oil production by about 90,000 barrels a day.

A spokesman says efforts are being made to restore output.  No group has claimed responsibility for the attack.

The oil-producing Niger Delta region is regularly subjected to attacks both by criminals who are looking to make money and militants who are demanding that residents in southern Nigeria get a bigger share of the country's oil wealth.

Nigeria's crude oil output is down about 20 percent since late 2005, when militants began their campaign against the government and oil companies.  Before the violence, Nigeria was pumping about 2.5 million barrels of oil per day.

Some information for this report was provided by AFP.