Four expatriates and two Nigerians working for the South Korean construction firm Hyundai Heavy Industries Limited were kidnapped Monday in southern Nigeria, police said.

"Some armed men went to a Korean company, Hyundai, located in a forest by the Atlantic Ocean beach, and kidnapped four expatriates and two Nigerians," police spokesman Fidelis Odunna told AFP.

A fisherman, Tare Kolugha, said he witnessed the abductions as the workers were travelling to their workplace in southern Bayelsa state.

"We were on the waters fishing close to Odioma Creek when we noticed two ... boats with armed men (who) shot into the air and stopped the passenger boat conveying the Hyundai workers to their yard," Kolugha told AFP.

Gangs have frequently abducted expatriates working in the oil-producing Niger Delta in ransom kidnappings, but Nigerians from wealthy families have increasingly become their targets in recent years.

A 2009 amnesty deal in the Niger Delta led to a sharp drop in unrest in the region, but criminality remains widespread.

Late last month, two foreigners who were identified by the local media as Lebanese were kidnapped at a work site in the region.

And the 83-year-old mother of the Nigerian finance minister, Ngozi Okonjo-Iweala, was kidnapped from her home in Delta state and held captive for five days before being freed on Friday.