Armed gunmen seized two expatriate oil workers -- an American and a Briton -- during a raid Thursday on a Norwegian oil services ship off Nigeria's southern coast, oil company officials said.

The two were seized before dawn from a vessel belonging to Oslo, Norway-based Petroleum Geo-Services, company official Christopher Mollerlokk said.

"I can confirm that two employees were kidnapped. One is a U.K. national and the other an American," Mollerlokk said by telephone from Oslo. He gave no further details.

A security official who works for a large multinational oil company in Nigeria said gunmen raided the vessel off the coast of Bayelsa State and sped away in boats with the hostages. He said the boat was surveying potential oil fields at the time. He declined to be named because he is not authorized to speak to the media.

Few details were available and government officials could not immediately be reached for comment.

There was no claim of responsibility and it was not known who carried out the attack.

Militants in Nigeria have launched a wave of kidnappings and attacks on oil facilities since the beginning of this year, forcing a 25 percent decrease in crude oil production, adding to upward pressure on world oil prices. Nigeria traditionally produces about 2.5 million barrels of oil daily, more than any other country in Africa.

The militants say they are fighting for a greater share of oil wealth from oil companies and the government. They say people in the area remain impoverished.

Dozens of foreigners have been kidnapped this year. The hostage-takings have usually ended peacefully, with those seized being set free after negotiations.

It was the second hostage-taking involving Petroleum Geo-Services in recent months. In June, two Filipinos working for the Norwegian oil-field services company were held hostage for five days by unidentified gunmen. They were taken captive as they inspected barges in the Niger Delta, where most of this African oil giant's petroleum is pumped. Both were released unharmed.