(Adds release confirmed in paragraph 8)

By Tom Ashby

LAGOS, May 3 (Reuters) - Gunmen kidnapped 20 foreign workers in three attacks in Nigeria's oil-producing Niger Delta region but eight were freed within hours after a mix-up, authorities and militants said on Thursday.

Militants from the Movement for the Emancipation of the Niger Delta (MEND) abducted eight workers from an Italian-operated offshore oilfield but released them on instructions of the group's leader.

World oil prices rose on news of the kidnappings, but fell sharply after MEND said it was releasing the captives.

Separately, three Koreans and eight Filipinos were seized from a power plant construction site in Rivers State by suspected ransom seekers, diplomats said.

A Dutchman was also taken from a bar near Warri in Delta State on Wednesday night, taking the total number of foreigners kidnapped in the region since Tuesday to 26.

MEND's leader, who uses the pseudonym Jomo Gbomo, told Reuters he ordered the release of the eight oil workers because he did not want any more hostages, having taken six from a U.S.-operated oilfield on Tuesday.

"We did not intend to take any more hostages, at least not just now," he said, declining to elaborate further on the reasons for the apparent volte-face.

A state government spokesman confirmed all eight were released on Thursday afternoon.

It was not immediately clear if Saipem <SPMI.MI> and SBM Offshore <SBMO.AS>, the operators of the 50,000 barrel-per-day Okono/Okpoho oilfield, shut off production because of the raid.

OUTPUT DOWN

The seizures are the latest in a series of militant raids on the oil industry since February last year that have led to oil output being reduced by 600,000 barrels per day, or a fifth of Nigerian production capacity.

The three Koreans taken in the power plant attack were senior managers of Daewoo Engineering who had just arrived in Nigeria, a security source in Nigeria said. Gbomo said MEND was not involved in the power plant raid.

One of the Korean hostages called his employer to let the company know they were safe, a company spokesman said.

Kidnappings of foreigners have increased sharply in the past 12 months in the world's eighth largest oil exporter, where militancy is fuelled by poverty, lawlessness, corruption and struggles for control of a lucrative oil theft business.

Hostages are almost always treated well and released unharmed.

On Tuesday, MEND seized six foreign oil workers from an offshore oilfield operated by Chevron <CVX.N>. The company reduced oil output there by 15,000 barrels per day.

The group said those kidnappings were to serve as a denial of reports that it supported Nigerian president-elect Umaru Yar'Adua and his deputy Goodluck Jonathan, who is also a state governor from the delta region.

MEND, which says it is fighting for more regional autonomy in the delta, was also behind the attacks in February 2006 that shut 600,000 barrels per day of oil production, mostly operated by Royal Dutch Shell <RDSa.L>.

Shell said on Thursday it was preparing to restart those oilfields, despite MEND's warnings to stay away.

(Additional reporting by Austin Ekeinde in Port Harcourt and Jack Kim, Jessica Kim and Kim Yeon-hee in Seoul)

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