Armed militants attacked targets in Nigeria's main oil industry center of Port Harcourt on Tuesday, leaving 13 people dead, a military spokesman said.

Bands of armed men invaded the city in the morning, attacking two police stations and raiding the lobby of a major hotel. Four policemen, three civilians and six attackers were killed, said Lt. Col. Sagir Musa, spokesman of the military task force in charge of security in Nigeria's troubled oil region.

The Niger Delta Vigilante Movement, led by militia leader Ateke Tom, claimed responsibility for the attack, the group spokesman Richard Akinaka told The Associated Press by telephone.

The group's strongholds in the creeks surrounding Port Harcourt have come under military bombardment in recent days. On Sunday, military planes bombed suspected training camps thought to be run by the militia group in mangrove swamps and creeks in the Okirika district, south of the city.

Tom later threatened reprisal attacks on the oil hub, where major Western oil companies have their operational bases.

The group is one of several armed movements active in the southern Niger Delta oil-producing region. Nigeria is Africa's leading oil producer, and fifth-biggest source of U.S. oil imports.

The attacks have cut the country's oil exports of 2.5 million daily by more than 20 percent in the last two years, and have added to the upward pressure on global oil prices.

Some of the groups claim to be fighting for increased access to oil wealth for inhabitants of the Niger Delta, who remain desperately poor despite the huge wealth pumped from their backyards. Others groups have simply targeted Western oil companies, seizing oil workers in exchange for ransom.

More than 200 foreign oil workers have been seized in the region since a new upsurge of attacks two years ago. Most were released unharmed after payment of ransom.