At least five Nigerian soldiers have been killed in a massive raid on a convoy of barges carrying fuel in the oil-producing Niger Delta.
Some 25 workers for the oil firm Shell were taken hostage, while nine more soldiers are missing, sources say.
About 70 armed men staged the attack in five speedboats and destroyed two military patrol boats.
A coalition of militant groups has said it carried out the raid and demanded the release of one of its leaders.
However, one of the groups supposed to be in the coalition, the Movement for the Emancipation of the Niger Delta, has denied involvement.
The self-styled coalition demanded the release of Mujahid Dokubo-Asari, who's been in prison for a year awaiting trial for treason.
But some analysts say stealing fuel and extorting money in return for the hostages was the primary motivation.
Election fears
The BBC's Alex Last in Lagos says in the Niger Delta the line between criminal gangs and the more political militant groups is often blurred.
He says gunmen, politicians and military officers are said to collude in the black market for oil.
Our correspondent says the incident highlights the continued vulnerability of the oil industry despite government pledges to crackdown on armed groups in the Delta.
Although there has not been a new wave of attacks on the actual oil infrastructure for some months, he says oil companies are bracing themselves for further violence in the Delta as competing politicians try to use the armed groups to secure victory in next year's elections.
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