Abuja, Nigeria ? Gunmen in speedboats attacked a Shell gas plant early Wednesday, sparking a firefight and kidnapping five South Korean contractors in the latest violence to hit Africa’s leading crude producer.

Six Nigerian soldiers and one of the assailants were killed in the raid, according to the Movement for the Emancipation for the Niger Delta, which claimed responsibility.

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

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 revenue.

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