The Movement for the Emancipation of the Niger Delta (MEND) claimed responsibility for the abductions and the attack, demanding in an email statement the release of a jailed militia leader in exchange for the Koreans freedom.
"The government of Nigeria may be interested more in a prisoner exchange rather than releasing the person whose release we have demanded," MEND said in an emailed statement.
The group has repeatedly called for the release of Asari, the leader of the illegal Niger Delta People's Volunteer Force (NDPVF), who was arrested in September last year and is standing trial for treason. He has repeatedly used his court hearings to accuse oil companies of exploiting the Ijaw people of the Niger Delta, and to demand control of the region's oilfields.
The attackers, armed with rocket launchers, approached the Royal Dutch Shell plant near Port Harcourt by boat and overpowered Nigerian soldiers and policemen who were guarding the site. Several security forces were killed in a fire fight.
As the militants left, with the Koreans, they came under attack from four Nigerian army boats. MEND says they sank one of the boats, killing at least five of its six occupants. A security source said that nine navy staff and a policeman were killed. Military spokesmen confirmed four dead. MEND confirmed that one of its fighters was killed and two injured.
Safe
The group’s email statement said the five Koreans were taken to a MEND base where they were safe and would remain so unless attacked, "As long as the units holding these individuals do not come under attack, no harm will come to the prisoners. We do not kill those fortunate to be captured by our fighters," it said.
But the militants warned that more violence will come, "In the next few weeks our attacks will increase (in) frequency with the destruction of several facilities of crucial importance to the oil industry," advising workers to leave the oil-producing wetlands region.
In South Korea, a Foreign Ministry official confirmed that three of the kidnapped men worked for Daewoo Engineering and Construction Co and that the other two nationals were with Korea Gas Corp.
The attack forced Shell to shut down its production of natural gas, but said it had no impact on oil flows from the area. "The gas plant has been shut down pending assessment of any damage, resulting in the loss of some 70 million standard cubic feet per day of gas production." Shell said in a statement.
Growing unrest
The attack comes three days after eight foreign oil workers were released by a different group of kidnappers, and is the latest sign of rising militancy in Africa's top oil producer. Apart from the five South Koreans, 24 expatriates working for oil firms have been kidnapped since the beginning of the year. All have been released unharmed.
MEND, whose leadership is unknown, is pressing for more local power over the Niger Delta's immense oil resources and has said that it aims to bring the OPEC nation's exports to a complete halt.
"Oil service companies in the Niger Delta which have been clamouring for contracts on the repair of pipelines we previously destroyed should rethink this foolishness. Workers for such daring companies will be executed if caught at those sites," its statement said.
Nigeria is Africa's largest oil producer and the world's sixth exporter with an average of 2.6 million barrels a day, most of which is derived from the Niger Delta area. Poverty, lawlessness, corruption and struggles for control of lucrative oil smuggling businesses fuel unrest in the delta, a maze of mangrove-lined creeks the size of England.