PORT HARCOURT, Nigeria (AP) _ Gunmen kidnapped four Norwegian and Ukrainian workers from a boat heading to oil rigs in the latest violence targeting the petroleum industry in Nigeria, the continent's largest exporter of crude oil, officials said Wednesday.

The two Norwegians and two Ukrainians were taken hostage in the Gulf of Guinea late Tuesday during a raid on a supply vessel owned by Norwegian shipping firm Trico Supply, company spokesman Bjorn Endresen said.

Trico Marine Services Inc., the company that owns Trico Supply, said it had made contact with the kidnapped Norwegians and negotiations had begun for their release.

``They are well,'' the company said in a statement.

There was no immediate word on the fate of the Ukrainians.

Andy Oputa, a security official for Nigeria's Bayelsa State, confirmed the kidnappings and said negotiators had been sent to the Niger Delta region, where most of the crude is pumped. Another state official said the capture took place nearly 30 miles from shore, with gunmen boarding the ship as it headed to oil rigs off the West African coast.

Endresen said 11 other crew members were aboard the ship at the time of the kidnapping but he did not give more details.

``All I can tell you is that we're working hard, putting all of our efforts to solving this case,'' he said.

Norwegian Foreign Ministry spokesman Frode Overland Andersen declined to comment on whether any ransom demands were made. He said the Norwegian Embassy in Abuja was working with local authorities to obtain more details on the abductions.

The kidnappings came less than a week after militants wearing camouflage uniforms took a German oil industry worker hostage, spiriting him away on a boat into Nigeria's troubled oil-rich delta region. A group calling itself the Movement for the Niger Delta People has claimed responsibility for that kidnapping, but police said the group was unknown.

Militants have kidnapped oil workers to bargain for a greater share of the wealth from Africa's largest crude producer. They argue that residents remain deeply impoverished and benefit little from oil wealth while government officials and oil companies grow rich.

More than 30 workers have been taken this year, including three from the oil-producing hub of Port Harcourt. Kidnappings and attacks have forced a nearly 20 percent reduction of Nigeria's usual 2.5 million barrel daily production, helping send crude prices soaring in international markets.

The vast majority of kidnappings end peacefully.

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Associated Press writer Jaime Espantaleon in Oslo, Norway, contributed to this report.