Nigerian police say suspected militants have launched a series of attacks in the southern city of Port Harcourt, killing at least 18 people and wounding several others.

Police say the militants attacked two police stations shortly after midnight Tuesday. They say at least three policemen were killed in a shootout at one station, along with seven civilians who were caught in the cross-fire.

A police official, Rivers State police commissioner Felix Ogbaudu, says the militants later opened fire at a Port Harcourt hotel, killing one security guard and wounding several other people. Gunmen also struck a night club in the area.

The police official says several militants were killed in the attacks but he did not provide exact numbers. There has been no claim of responsibility for the attacks.

Port Harcourt is the main city in Nigeria's volatile Niger Delta region, the center of the country's oil production.

Crime and violence have been running high in the Niger Delta since the end of 2005, when militants stepped up a campaign to direct more of the region's oil wealth to local residents.

Attacks and kidnappings have cut Nigeria's oil output since then by about 20 percent. Some of the violence been the work of criminal gangs seeking payment from the government or oil companies.

Despite the violence, Nigeria remains sub-Saharan Africa's largest oil producer, pumping more than two million barrels per day.

Some information for this report was provided by Reuters.