ABUJA, Nigeria (AP) — At least four Nigerian soldiers were killed while two South Koreans were abducted during an attack by gunmen in Nigeria’s oil-rich southern Rivers state, authorities said.

The gunmen ambushed a convoy escorting the Koreans on a work trip in the Ahoada East council area, resulting in a shootout, Maj. Jonah Danjuma, an army spokesman, said in a statement on Tuesday.

“Troops are currently combing the general area to fish out the perpetrators of this dastardly act. Additionally, efforts are ongoing to ensure that the unaccounted oil workers are found,” Danjuma said.

Abductions for ransom are common in parts of Nigeria, but it has been especially widespread in Rivers and other parts of the oil-rich Niger Delta region where many foreign companies in the oil and engineering sectors operate. The region also struggles with high poverty and hunger despite its natural resources.

Hostages are usually released after the payment of large ransoms, though security forces in the past have succeeded in freeing some.

Danjuma urged locals to provide any intelligence that would help in making arrests.