Nigerian militants in the oil-rich Niger Delta appear committed to making good on promises to cripple the country's petroleum industry, this time hijacking an oil vessel off the coast.

A Portuguese and a Ukrainian -- along with nine Nigerians -- were taken hostage when their vessel, belonging to Chevron Corp., was seized off the coast of the delta's de facto capital, Port Harcourt.

So far, no one has taken responsibility for the attack, which occurred late Tuesday. However, Nigeria's leading militant group, the Movement for the Emancipation of the Niger Delta, has denied carrying out the assault and denounced the unknown assailants in a statement.

The kidnappers have asked for $259,000 in local currency (30 million naira) in ransom for the captives.

The attack on the Chevron vessel was the latest in a series of violent acts aimed at derailing oil production in the delta.

Companies like Chevron, ExxonMobil and Nigeria's leading foreign producer Royal Dutch Shell have seen output and profits dwindle since militants pledged to ramp up attacks in the region. Earlier this year Shell lost 169,000 barrels per day in production when a militant attack forced the company to close one of its best-producing pipelines for repairs.

Before that, production was already down by 20 percent nationwide to 2 million barrels per day, a shortfall blamed directly on MEND and other groups, contributing to an international spike in oil prices over the last few weeks.

Shell might not be able to honor production contracts because of decreased production levels due to attacks on its installations. MEND has claimed responsibility for at least three attacks on Shell installations, including a facility at the Bonny terminal, causing the 169,000 bpd shortfall.

"The recent slump in Nigeria's oil output may have been disastrous for Africa's largest oil producer, but problems could worsen," an official told Nigeria's Leadership newspaper, adding that nearby Angola, whose oil industry is booming, could capitalize on Nigeria's chronic violence and production shortfalls and eventually assume the mantle of Africa's largest oil producer.

MEND's commitment to thwarting full-capacity production in the delta doesn't help. Its leaders said the group was stepping up attacks because of the arrest of one of its most prominent leaders, who is facing trial by a secret commission on several charges including weapons trafficking and treason.

MEND and other militant groups have been blamed for hundreds of kidnappings since violence in the delta began in 2005. Increased violence against oil operations in the delta has caused significant drops in the country's oil output, according to the Nigerian government and independent accounts.

Since the 1970s, Nigeria, Africa's No. 1 oil producer, has pumped more than $300 billion worth of crude from the southern delta states, according to estimates. High unemployment in the delta, environmental degradation due to oil and gas extraction, and a lack of basic resources such as fresh water and electricity have angered the region's youth, who have taken up arms, many times supplied by political leaders, and formed militant groups and local gangs.

The militants have called for a more equitable distribution of the country's oil wealth.

Nigerian President Umaru Yar'Adua has reached out to the militants and asked for them to give his administration time to tackle the problems of the delta. The proposed reforms include changes to the Nigerian economy, particularly its petroleum sector, which generates up to 95 percent of the country's revenue.

But MEND and other militant attackers seem reticent to lay down their arms based on promises from Nigerian leaders, though they have called on American leaders past and present to intervene in the delta.

Former U.S. President Jimmy Carter was asked last week to mediate talks between rebels and the government to end hostilities in the oil-rich Niger Delta. MEND is said to be weighing a reported cease-fire appeal by Democratic presidential hopeful Barack Obama.

"We are ready to call off all hostilities and hold a temporary cease-fire in honor of President Carter should the Nigerian Government accept," read a statement purportedly from the Movement for the Emancipation of the Niger Delta, Nigerian news sources reported.

Carter, who unsuccessfully mediated talks between Nigeria and militant groups in 1999, has not said publicly whether he would accept the invitation.

Meanwhile, Obama, according to MEND, has called for a cease-fire in the delta. A MEND spokesman said the Democratic candidate is "someone we respect and hold in high esteem" and that his supposed call for a cessation of hostilities is something they are considering.

Obama officials have not confirmed whether the senator did speak out on the delta violence, which analysts attribute in part to oil prices reaching record highs.

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