LAGOS Gunmen kidnapped four American oil workers from a barge off the Nigerian coast on Wednesday in the 10th attack on Western oil facilities in nine days in Africa's top producer.
Rebels from the Movement for the Emancipation of the Niger Delta (MEND) said they had called for a month of "mayhem" before a new government is inaugurated in Nigeria on May 29, after disputed elections last month.
Militant attacks have already cut output in the world's eighth largest oil exporter by a quarter.
"The Global Cheyenne, a construction barge working in the Okan field, was attacked by unknown armed persons in speed boats," U.S. oil company Chevron said in a statement.
"Four expatriate American hostages were taken from the vessel and some government security forces suffered injuries during the attack."
No group claimed responsibility for the latest attack, which helped push London Brent crude oil futures near $66 a barrel on Wednesday morning. Prices eased later.
But MEND is at the vanguard of an armed insurrection to secure regional control over the delta's oil wealth, and militants see the transition to a new government on May 29 a chance to extract concessions.
About 100 foreigners have been kidnapped this year in the vast wetlands region, but most were released after their employers paid ransoms. The abduction of the Americans takes the total number of foreigners now in captivity to 13.
The barge, operated by U.S. contractor Global Industries, was laying pipelines for the U.S. giant, and oil supplies were unaffected by that attack.
Chevron is facing another crisis at the nearby Abiteye oilfield, which was occupied by villagers demanding oil spill compensation on Monday, halting 42,000 barrels per day (bpd).
On Tuesday, MEND fighters blew up three oil pipelines near the Brass export terminal, forcing Italian oil firm Eni to reduce output by 98,000 bpd and issue a legal notice to default on oil sales, the company said.
"Groups are responding to a call we made to unleash mayhem in the delta," said the MEND spokesman, who uses the pseudonym Jomo Gbomo, in an e-mail to Reuters. "We will pause temporarily in about one month's time."
Gbomo said he had instructed fighters to inflict more damage on the Brass pipelines and possibly attack the terminal itself.
The MEND has told oil workers to leave the region and vowed to bring Nigerian exports to a complete halt to press its case for more autonomy from the central government. It also wants the release of two jailed leaders from the delta and $1.5 billion in compensation to villages for decades of oil spills.
Elections last month, which were marred by widespread abuses, gave Umaru Yar'Adua of the ruling People's Democratic Party (PDP) a landslide victory.
Yar'Adua picked Goodluck Jonathan, the governor of Bayelsa State in the delta, as his running mate in an bid to ease tensions in the region. MEND has dismissed this overture and accused Jonathan of betraying the militant cause.
"Forgetting that we were instrumental to his assuming that office, he calls us criminals and terrorists," Gbomo said.
Energy Minister Edmund Daukoru said he expected the violence to subside after Yar'Adua's inauguration.
"There is some unfinished business in the Niger Delta. This is simply a signal, not the start of something that will be sustained," he told Reuters in London.
(additional reporting by Milan bureau and Alex Lawler in London)