Gunmen attacked a Chevron Corp. oil pipeline in Nigeria's restive Niger Delta, causing an unknown amount of damage, a government spokesman said Saturday.

Linus Chima, a Delta State spokesman, said the gunmen damaged the Makaraba pipeline southwest of Warri. That pipeline transports crude oil out of one of Chevron's seven swamp fields in the area, which produced 77,000 barrels of oil per day in 2008.

Chima said government officials still were investigating the attack, which apparently occurred early Friday morning. Chevron previously pulled out of the region in 2003 over vandalism and attacks by local militants, but returned in 2007.

Scott Walker, a spokesman for San Ramon, Calif.-based Chevron, said early Saturday morning that he could not confirm the attack on the pipeline operated by its Nigerian subsidiary.

"We are not speculating on any comment while investigations are under way," Walker said.

Lt. Col. Timothy Antigha, a military spokesman, said no one had claimed responsibility for the attack. He said soldiers began working their way along the pipeline system Saturday to find damage, though he stressed he had no independent confirmation of an attack.

"We are doing our checks along with the oil company," Antigha said.

Militants in the Niger Delta attacked the same area in June during a wave of violence before some began laying down their arms as part of a government amnesty program. The Movement for the Emancipation of the Niger Delta, the main militant group in the region, has not mentioned the Friday attack. However, the group said in December it would consider its cease-fire agreement with the government void for 30 days.

The attack comes after soldiers shot two contract workers dead and injured four others at Chevron's Escravos gas project nearby. The soldiers opened fire after buses carrying the contract workers out of the plant blocked them from entering the plant's property. Workers responded by attacking the soldiers and setting fire to several buildings there.

Chima said he did not know whether the shooting Monday had any part in the pipeline attack.

Militants in the Niger Delta have attacked pipelines, kidnapped petroleum company employees and fought government troops since January 2006. They demand that the federal government send more oil-industry funds to Nigeria's southern region, which remains poor despite five decades of oil production.

That violence has cut Nigeria's oil production by about 1 million barrels a day, allowing Angola to surge ahead as Africa's top oil producer.