Gunmen attacked a boat carrying oil workers in Nigeria's violence-wracked southern delta region, killing two Americans and at least three Nigerians, company and army officials said Saturday.

A third American, an employee of oil giant ChevronTexaco (search), was in stable condition after being shot during the Friday afternoon attack on the Benin River near the southern city of Warri, ChevronTexaco spokesman Deji Haastrup said.

The gunmen opened fire on the boat's passengers after navy personnel guarding the oil workers refused to give up their arms, Haastrup said.

The motive for the attack was unclear, but ethnic fighters and ordinary criminals regularly sabotage multinational oil facilities and take oil workers hostage in the Niger Delta (search) to demand payoffs from the companies.

Authorities have recovered the bodies of the two slain Americans, who were employed by a firm contracted by the U.S. oil giant, said Haastrup. He declined to identify the contractor.

"My heart goes out to the families of the victims of this tragic event," said Jay Prior, chairman and managing director of ChevronTexaco's Nigerian subsidiary.

"The safety and security of all employees will continue to be our top priority," he said in a written statement.

Also retrieved were the remains of three Nigerians: two navy soldiers and one boat crew member identified as a contract worker for international oil consulting firm Willbros (search), military officials said. Two other Nigerians remain missing, they said.

The killings were confirmed by Maj. Said Ahmed, commander of a Nigerian army task force charged with protecting oil installations in the volatile, petroleum-rich region.

Ahmed said Nigeria's army is investigating whether the attack is linked to growing ethnic tensions in the area following efforts by ethnic Itsekiris to return to villages they fled during an outbreak of fighting last year.

The oil workers had been sent to inspect ChevronTexaco oil and gas facilities at Dibi and Olero Creek, among dozens of installations abandoned by the company since the outbreak of ethnic violence in March 2003, company officials said.

"We have suspended activities relating to the restoration of swamp facilities," Haastrup, the spokesman, said on Saturday. "We will only operate if it is safe."

In all, ChevronTexaco's idled installations would normally produce 140,000 barrels of oil daily. With production of 2 million barrels a day, Nigeria is Africa's largest oil producer and the fifth biggest source of U.S. oil imports.

Michael W. Collier, a Houston-based spokesman for Willbros confirmed that one of the slain Nigerians worked for that company. Another Nigerian employee of the company was still missing, Collier said.

He said Willbros did not employ the slain Americans.

Friday's shooting came as the two main ethnic groups in the region — the Ijaws and Itsekiris — threatened this week to escalate hostilities after an attack on a boat that killed 10 people.

Ahmed said Itsekiri gunmen were believed responsible for that attack.

Last weekend, naval forces killed five Ijaw assailants carrying shotguns as the attackers tried to storm an oil facility owned by Agip, a division of Italy's ENI SpA.