PORT HARCOURT, Nigeria (Reuters) - The Nigerian army attacked a suspected criminal hide-out in the anarchic oil-producing Niger Delta on Friday using helicopter gunships and ground troops, an army spokesman said.

The target was a criminal gang suspected of murdering three chiefs and two other people from Ogbogoro, a waterside community on the outskirts of Port Harcourt, the delta's biggest and most violent city, spokesman Sagir Musa said.

"The criminals who killed the chiefs and civilians recently have regrouped in the thick forest near the community. So we went there to root out the criminals," Musa said.

He said soldiers were firing at the suspected hide-out and dropping bombs from several helicopters while ground troops were surrounding the area. He declined to comment on casualties.

Private security contractors said flights in and out of a Port Harcourt air force base that is also used by commercial carriers were grounded because of the attack.

Violence has spiralled out of control in the Niger Delta, especially in Port Harcourt which has been under a dusk-to-dawn curfew since clashes involving rival gangs and troops killed dozens of people last month.

Security in the delta, which accounts for Nigeria's 2.2 million barrels per day in oil output, deteriorated in early 2006 when armed rebels demanding local control of oil revenues started blowing up pipelines and kidnapping foreign oil workers.

The raids shut down about a fifth of Nigeria's production capacity, contributing to spikes in world oil prices.

Over time the crisis escalated into a crime wave that the security forces have been unable to quell despite several heavy-handed operations like the one at Ogbogoro.

Political militants have observed a truce since President Umaru Yar'Adua took office on May 29 promising urgent action to develop the impoverished delta and address the rebels' demands.

But ransom seekers, armed robbers and violent gangs have all stepped up their attacks. Kidnappers have snatched toddlers and elderly people, gang fighters have fired indiscriminately at residents of Port Harcourt and armed robberies have multiplied.

Yar'Adua says he has a two-pronged strategy to resolve the crisis. One element is a 15-year development plan to address the underlying problems of poverty and unemployment, while the other is negotiations with the rebels.