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. (NYSE:CVX), 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.