U.S.-based oil company Chevron says unidentified assailants have attacked one of its key oil supply pipelines in southern Nigeria, forcing a shutdown in crude production.

Nigerian military officials say the attack took place late Thursday near the Escravos area of the Niger Delta.

Chevron confirmed the attack in a statement Saturday, saying it shut off its onshore production following the pipeline breach to protect the environment.  It did not say how much production would be lost.

The region's main militant group, the Movement for the Emancipation of the Niger Delta, says it was contacted today by a group of youths, who claimed responsibility for the attack.

Nigerian President Umaru Yar'Adua has ordered the country's armed forces to boost security in the region following a separate attack on an offshore oil field Thursday.

Government spokesman Segun Adeniyi says a major military crackdown is inevitable following the militant attack on the Royal Dutch Shell oil installation.

The Movement for the Emancipation of the Niger Delta claimed responsibility for that rare offshore attack on the Bonga field, 120 kilometers out to sea.  

Militants began attacking oil facilities in the Niger Delta in late 2005 to demand that more oil revenue be directed to poor local residents.

Some information for this report provided by AP and Reuters.