Several people are feared dead after the military attacked armed gangs in Nigeria's oil city of Port Harcourt.
The gangs are thought to be linked to Niger Delta's militants
Reports suggested both militants and military personnel were killed in subsequent fighting, but officials have not yet confirmed any fatalities.
It follows clashes that raged last week between powerful rival armed gangs that left at least 15 people dead.
Militant groups in the Niger Delta say they are fighting to gain more local control over the region's oil supplies.
Residents in some parts of Port Harcourt described blazing buildings and streets deserted except for men on motorbikes carrying automatic weapons.
Others said they had seen helicopter gunships firing into the city, as well as armoured personnel carriers near government offices.
The fighting followed an operation by the military against the base of one of the major gang leaders in the city, Soboma George.
Major Sagir Musa said the gang leader was "linked to different kinds of atrocities in terms of hostage-taking, killing and maiming of innocent citizens".
"Members of the joint taskforce made a surprise attack on the hideout, and in the process, Soboma George is suspected to have been killed," he said.
But the militant group Movement for the Emancipation of the Niger Delta said Soboma George was "alive and well".
The army said it arrested a number of militants during the raid, but he was not among them.
For most of last week rival gangs fought pitched battles around the city leaving many dead, mostly bystanders.
The authorities say it is a turf war.
The gangs that run criminal rackets are large and have plenty of weapons.
The BBC's Alex Last in Lagos says there is also a suspicion that others may have an interest in the fighting.
Most of the armed gangs had close links to politicians who employed them to help rig elections.
Gangs, money and politics have long been a dangerous combination in the Niger Delta, he says.
In the aftermath of April's polls, he says it seems all sides are trying to exert their control, whether it be the gangs, the politicians or the security forces directed by the new government.