PORT HARCOURT, Nigeria - Gunmen jumped from a minibus in Nigeria's southern oil center and killed two police officers Thursday in the latest violence to strike the petroleum-producing region, officials said.

The attack in the city of Port Harcourt took place near the offices of major international oil and construction companies, said Rivers State police spokeswoman Irejua Barasua.

Militants demanding greater control over oil in the region have stepped up attacks in recent weeks in Africa's biggest oil producer.

Early Tuesday, attackers with assault rifles and rocket-propelled grenades stormed a boat and seized the four American workers, two industry officials said, speaking on condition of anonymity due to company restrictions on dealing with the media. Chevron Corp. spokesman Femi Odumabo said the four were subcontractors with U.S. citizenship.

That same day, militants staged coordinated bombings on three pipelines in the region, the most damaging assault on the country's vital oil infrastructure in more than a year. An Italian oil company said daily crude production had been cut by nearly 100,000 barrels a day by the blasts.

AP