At least two people have been killed in a clash between the Nigerian navy and suspected militants in the oil-rich Niger Delta, security sources say.
The militants had been observing a ceasefire
It occurred near an offshore oilfield operated by the Shell oil company.
On Tuesday, six foreign oil workers were released after being seized during a raid on the oil platform.
The BBC's Alex Last in Nigeria says it is the latest violence to hit the area since a militant faction pledged to renew attacks on the oil industry.
Militant groups had been observing a five-month ceasefire on attacks against the oil industry to allow for peace talks with the government.
Our correspondent says just hours after six hostages were freed by militants, gunmen in the Niger Delta took to the water again.
At night, their speedboats came across a Nigerian naval patrol assigned to protect an oilfield operated by the Shell oil company.
A fight ensued and at least two people have been killed and three others were injured.
There are reports of military causalities but details are sketchy.
It is the latest sign that the ceasefire in the Niger Delta is unravelling, our reporter says.
Five months ago, militants had publicly declared a cessation of attacks against the oil industry to allow for peace talks with the new government.
But recently, one militant faction called off the truce, citing the lack of progress at peace talks and the detention of one of its leaders in Angola.
Others say the reason for the renewed violence is simply that some militants feel that they have not got their share of the cash and contracts quietly offered by the government and oil companies to shore up the peace process.
It was hoped the recent calm would enable the oil companies to restore the huge chunk of Nigeria's oil production, which had been cut by militant attacks last year, thus also restoring a huge chunk of oil revenues to the government.
But without a ceasefire, our correspondent says, this becomes a lot more difficult.