Man walks along oil pipelines belonging to Italian oil company Agip in Obrikom, Nigeria (File)

The group says its fighters destroyed three large oil pipelines in Bayelsa state of Southern Nigeria. In an e-mail message to journalists, MEND's spokesman Jomo Gbomo said the attack cut the power at a nearby export facility owned by Italian oil company Agip.

The group vows to attack pipelines, platforms and support vehicles in the coming days to disrupt oil exports.

A spokesman for the Bayelsa state government, Ekiyol Wilson, calls the attack a criminal act.

"This morning, we just woke to hear that they have attacked oil facilities in Brass and Akasa," he said.  "It is pure criminality. It has been confirmed. It has been confirmed through our sources. Government will do something about it. That is government reaction. Government is looking into it and government will get to the bottom of the matter. That is all we can say."

MEND, which is considered the best-organized militant group in Nigeria, also released pictures of foreign oil workers seized a week ago from an offshore oil facility.

The hostages, six of them, were casually dressed and looked well, except perhaps a bit tense.

The group, which claims to be fighting for a larger share of the country's oil wealth to go to local communities, has carried out kidnappings, car bombings and other attacks in its campaign to cripple the oil industry.

MEND also seeks the release of two leaders on trial on treason and corruption charges.

Oil exports are down about 25 percent and analysts predict more cuts if the violence escalates. Nigeria is Africa's largest oil exporter.