Amnesty International, an influential human rights group, is calling on oil giants Chevron and Shell to examine their possible connections to deadly violence in Nigeria.

In a report scheduled for release today, Amnesty asks San Ramon's Chevron to investigate an incident at the company's Escravos oil terminal in February that left at least one protester dead, according to news reports.

On Feb. 4, Nigerian soldiers reportedly fired on protesters at Escravos, some of whom were carrying signs asking for jobs. The protesters had cut through the security fence guarding the compound and smashed windows and helicopter windscreens. One protester reportedly died of a gunshot wound.

Chevron says protesters attacked the facility.

Amnesty also wants Shell to investigate allegations that one of its Nigerian subcontractors hired for security a "vigilante group" suspected of killing 12 people. Nigerian troops reportedly destroyed a village and killed 17 people while hunting for the vigilantes.

The carefully worded report stops short of calling Chevron and Shell complicit in the violence. But it argues that international oil companies working in Nigeria have an obligation to ensure that local forces protecting them don't violate human rights.

"With Chevron, we want to see whether they followed their own principles in this incident," said Mila Rosenthal, director of Amnesty's business and human rights program.

Both Chevron and Shell dismissed Amnesty's appeal Wednesday.

"Their statements are unfair, they're unbalanced and they're anything but objective," said Chevron spokesman Jeff Moore.

Moore said Chevron already conducted an internal review of the incident at the Escravos facility. He declined to discuss details, but said Chevron met with Amnesty representatives at Escravos and showed them photos and documents "demonstrating that this was an attack on the facility."

He added that Amnesty ignored steps Chevron has taken to help Nigerian communities. During a 2003 outburst of fighting among ethnic groups, for example, Chevron airlifted about 3,000 Nigerians to safety. Chevron also notes that 90 percent of its employees in the country are Nigerian.

"The report doesn't take into account any of the positive things Chevron Nigeria Ltd. is doing," Moore said.

For its part, Shell rejected the alleged link to vigilantes. A company spokeswoman also said the London firm had no connection to the reported attack on the village, when troops looked for the vigilantes.

"No (Shell) staff and no contractor staff were present when the reported incident took place, and neither (Shell) nor its contractor employed any armed group," said company press officer Bianca Ruakere.

The Amnesty report focuses on the oil-rich Niger Delta, a region gripped by poverty, pollution and interethnic rivalries.

Insurgents there have threatened to seize or destroy oil facilities to starve the government of cash. Ethnic groups have fought over control of territory. Poor Nigerians have staged protests at oil facilities to demand jobs.

Oil companies there are protected by Nigerian soldiers, who have been accused of responding with excessive force to protests. In some cases, they have allegedly attacked protesters' villages.