The Nigerian military says it has fought a fierce gun battle with heavily armed militants in the Niger Delta.
Militant attacks have led to a 20% drop in Nigeria's oil exports
Thirty speed boats each carrying 15 militants attacked a petrol tanker demanding fuel, an army source says.
The group, which is demanding a greater share of the region's oil wealth, says the military initiated the attack.
Wednesday's incident took place in an area known as a militant stronghold and near where it is suspected three foreign hostages are being held.
In a statement, the Movement for the Emancipation of the Niger Delta (Mend) said their forces were out patrolling the rivers and creeks when they were attacked in the broad mangrove-lined Escravos River by a total of seven navy patrol boats near the village of Okerenkoko in the western Niger Delta.
The militants, who were armed with rifles, machine guns and rocket-propelled grenades, said the gun battle lasted for 45 minutes.
However, military sources and an Ijaw leader says the militants attacked a petrol tanker because they needed fuel.
The vessel, which had a military escort, then sent out a distress signal and military reinforcements were despatched.
Both sides agree that the ensuing fire fight was fierce and it was deadly
The militants say they killed 13 government soldiers; the military says its forces killed many militants.
The BBC's Alex Last in Lagos says there have been no reports of further clashes on Thursday, but the increased tension in the area will be of concern to the negotiators trying to secure the release of the three foreign hostages.
Six hostages were released last week
They were among a group of nine oil workers seized during militant raids last month.
Six hostages were released last week, but two US citizens and one Briton are still being held and have now spent more than two weeks in captivity.
The militants have issued a long list of demands.
In particular, they want a pledge from the government that it will not retaliate once the hostages are released and a greater local control of revenue produced from the oil industry.
The recent unrest in the Delta region has led to a 20% drop in Nigeria's oil exports.