The Nigerian army has launched an offensive operation against the Niger Delta militants in the Delta oil hub after many appeals from the government to the militants.
Armed groups have claimed responsibility for a series of attacks on oil and gas pipelines in the southern region, reducing the country's oil output by 700,000 barrels day.
A special forces battalion moved against militant camps on Friday in an operation "aimed at getting rid of all forms of criminal activities", army spokesman Sani Usman said in a statement.
"In the course of the operation, five militants that attacked the troops were killed in action, while numerous others were injured and 23 suspects were arrested."
There was no immediate reaction from militant groups, which operate from hard-to-access creeks in the swampland.
The groups say they want a greater share of Nigeria's oil wealth to go to the impoverished region. Crude sales account for about 70 percent of Nigeria's government revenue and most of the oil comes from the Delta.
A similar military campaign in May drew sharp criticism from rights groups and residents who said soldiers had laid siege to villages, arrested civilians and raped women in an bid to force them hand over militants. The army denies this.
The government has been trying to broker a ceasefire but the militant scene is divided into small groups whose fighters, drawn from unemployed youths, are difficult to control even for their leaders.
On Thursday, Oil Minister Emmanuel Ibe Kachikwu met traditional leaders from the Delta to ask them to mediate in talks with militants but they said they wanted the army first to release prisoners taken during a previous sweep, an official has told Reuters.
The army in May arrested a group of school teenagers who community leaders say are not linked to militants.
A group calling itself Niger Delta Avengers, which has claimed several major attacks, said in a statement on Sunday they had agreed to a ceasefire to start a dialogue. Officials have refused to confirm this.