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.