4 police officers killed, 2 wounded near Nigerian oil center

Gunmen killed four police officers and wounded two others in an attack on a police outpost in Nigeria 's oil-rich southern delta, authorities said Monday. Unidentified assailants attacked a police outpost on an island near Port Harcourt , the largest city in Nigeria 's oil region on Sunday, police spokeswoman Ireju Barasua. She said the wounded were being treated at a hospital.

"Four bodies were found floating (in the water), while two other policemen were injured," Barasua said. She said they did not know who was responsible for the attack.

Militants have recently planted bombs and kidnapped foreign workers in the area around Port Harcourt as members of local communities use force to demand a greater share of Nigeria 's oil wealth. Last week, an American oil worker was killed by a gunman on a motorcycle in Port Harcourt , and three other foreign oil workers were kidnapped but they were quickly released. Kidnapping of oil workers is common in the delta. Hostages are usually released unharmed.

It can be unclear in the restive region when violence is tied to a political movement. One militant group the Movement for the Emancipation for the Niger Delta has said they set off recent car bombs at oil installations, but no one has claimed responsibility for the American's death. Nigeria is Africa 's largest oil producer, pumping about 2.5 million barrels a day. The west African nation also is the fifth-largest source of U.S. oil imports, reports the AP.