A militant group said Thursday that it will release five South Korean contract workers kidnapped from a Shell gas plant in Nigeria's oil delta in the latest attack against the African country's oil industry.

The Movement for the Emancipation of the Niger Delta, which claimed responsibility for Wednesday's abductions, said in a statement it would release the South Korean contractors by Thursday afternoon.

CountryWatch: Nigeria

It said its decision came in response to an appeal by Mujahid Dokubo-Asari, a prominent militant leader whose freedom they had demanded in exchange for the hostages' release.

Dokubo-Asari, who has called for autonomy for southern Nigerians, was jailed last year on treason charges. His release has been a top militant demand since they took up arms earlier this year. It was unclear how or where Dokubo-Asari made his appeal.

The hostages were contract workers at the Shell plant near Nigeria's oil hub of Port Harcourt, the Nigerian arm of Royal Dutch Shell PLC said. Shell said it has shut down the plant, which has capacity of 150 million cubic feet of gas a day.

MEND, the main militant group in Nigeria, has been responsible for a wave of attacks and abductions this year in the country's oil-rich southern delta. The militants say impoverished southern Nigerians aren't getting enough of the oil revenues.

Abductions are common in the volatile region, with most captives released unharmed.

MEND again warned the South Koreans' employer to leave the Niger Delta "or face even more drastic action" in the future. The group said Wednesday it would target "facilities of crucial importance to the oil industry" in attacks over the next few weeks.

A police spokesman in the Nigerian capital, Haz Iwendi, said the attackers arrived in 10 speedboats just after midnight.

The militants said they launched the assault at a delta outlet called Cawthorne Channel, burning a military houseboat used by security forces to defend the facility and killing some of its occupants.

The group said four Nigerian naval vessels launched a counterattack, sparking a battle that destroyed a military boat that had six soldiers aboard at the time.

Iwendi could not confirm any deaths, but said one policeman was in critical condition after being shot and four civilians were injured.

South Korea's Foreign Ministry said three of the kidnapped South Koreans work for Daewoo Engineering and Construction Co., and the other two are employed by the Korea Gas Corp. A Nigerian also was kidnapped, the ministry said.

Nigeria is Africa's leading oil exporter and the United States' fifth-largest supplier, usually exporting 2.5 million barrels daily. Militant attacks on oil pipelines and kidnappings have cut oil production by more than 20 percent this year and sent world oil prices soaring.

Last week, unidentified militants from southeastern Bayelsa state who were demanding jobs and money kidnapped six Britons, one American and one Canadian from an offshore oil platform. All were released unharmed days later.