A British oil worker has been killed and an Italian seriously wounded during an attempt to free seven workers seized by Nigerian militants.
A range of gangs and militant groups operate in southern Nigeria
A Nigerian navy spokesman told the BBC that the remaining five foreign hostages had been rescued unharmed, while two militants were killed.
The hostages - two Finns, an Italian, a Filipino, a Briton, a Pole and a Romanian - worked for Italian firm Eni.
Hostages are usually freed unharmed, often after ransoms are paid.
Oil workers are regularly abducted in Nigeria by gangs saying they want a bigger share of oil revenues.
The BBC's Alex Last in Lagos says the line between criminal gangs and political militants is often blurred.
He says there is a lot of pressure on Nigeria from oil companies and foreign embassies not to use force to release hostages, in case it endangers the lives of the workers.
The hostages were seized from a supply ship off the Niger Delta coast by 10 armed men on speedboats early on Wednesday morning.
Nigerian forces became involved in a shoot-out with the kidnappers during the rescue attempt.
"The Nigerian soldiers intercepted their ship and tried to free the people kidnapped," Gianni di Giovanni, a spokesman for Eni, told BBC News 24.
"In this situation, unfortunately, one of these colleagues was killed, and another was injured."
He said that all the rescued oil workers had now been taken to hospital in Port Harcourt.
One Nigerian solider was also killed during the operation, government and security officials told the Associated Press.
Eni was forced to shut down its operations at the Okono/Okpoho oilfield after the attack, an industry source told Reuters.
Nigerian navy Captain Obiora Madani told the BBC that several militants had been captured during the operation to rescue the kidnapped workers.
Eni said the remaining 76 members of the crew who were on the ship when it was first attacked are in good health and no damage to the ship was reported.
A series of attacks in Nigeria's main oil-producing region has cut output by about 25% this year.
Nigeria is among the world biggest oil producers but most of its people live in poverty.
In September, President Olusegun Obasanjo promised strong action to curb armed groups in the Niger Delta, but the attacks have continued.
Our correspondent says that this determination to take a tougher line has proved a risky policy with tragic consequences.
Oil companies are now bracing themselves for further violence in the Delta, he says, as competing politicians try to use the armed groups to secure victory in next year's elections.