Bands of armed men invaded the city on Tuesday 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, a spokesman for 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's spokesman Richard Akinaka told Associated Press.
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 the major western oil companies have 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 US oil imports.
The attacks have cut the country's oil exports, standing at 2.5 million barrels daily, by more than 20% 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. Other 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 attacks resumed two years ago. Most were released unharmed after payment of a ransom.
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