Five Nigerian soldiers have been killed in a raid on an oil field in which five South Koreans were kidnapped, a group of oil militants says.
Militant attacks have cut Nigeria's oil production by 25%
The Movement for the Emancipation of the Niger Delta (Mend) said it might swap the hostages for a local leader charged with treason.
Mend has staged several attacks in Nigeria's main oil producing region to demand more oil wealth for locals.
Eight oil workers were freed on Sunday, two days after they were seized.
Attacks on oil facilities have become increasingly common in Nigeria's lawless, impoverished Delta region, leading to a 25% cut in oil production by Africa's biggest exporter.
Armed groups have long complained that oil wealth has not been distributed equitably among the local population.
The BBC's Alex Last in Nigeria says there are fears that the violence will only get worse as next year's elections approach.
Mend said they had sunk an army boat, killing at least five soldiers, after they came under attack as they retreated into the creeks of the Niger Delta.
One militants was also killed, the group said.
There is no independent confirmation of the casualties, but Nigerian military sources say some soldiers have been reported missing.
The South Koreans were safe and would not be harmed unless they were attacked, Mend said.
Mend said the attack was a response to a court decision to deny bail to militant leader Mujahid Dokubo-Asari.
Mr Dokubo-Asari was arrested on treason charges last year. Mend has long demanded his release.
"We are of the opinion that the government of Nigeria may be interested more in a prisoner exchange rather than releasing the persons whose release we have demanded," Mend said.
Three of the kidnapped workers are from South Korean engineering firm Daewoo. The others work for the Korea Gas Corp.
"The Daewoo company is advised to close down its operations with immediate effect as a second attack will bring only death," Mend said.
The latest abduction took place just after midnight in Port Harcourt, Nigeria's oil hub, South Korea's foreign ministry said.
The ministry said a Nigerian had also been abducted.
Our correspondent says that many armed groups in the Niger Delta have links to competing local politicians and have been used to help win elections in the past.
So the fear is that violence in the Delta will rise as armed groups, political or otherwise, strive to exert their power, as campaigning starts for elections due next year.