Nairobi/Abuja  - Nigeria's main oil militant group, which last week issued an ultimatum to oil companies to leave the oil-producing Niger Delta, on Sunday said it had blown up two oil and gas pipelines.

"Already, two major trunk pipe and gas lines which were recently repaired have been blown up," Movement for the Emancipation of the Niger Delta (MEND) spokesman Jomo Gbomo said in an e-mailed statement. "This is just the tip of the series of attacks we plan to carry out."

No independent confirmation of the militants' claim was immediately available.

Hostilies between MEND and government troops have escalated since Wednesday, when clashes took broke out and an affiliate of the militant group seized the MV Spirit, a tanker chartered by the Nigerian National Petroleum Corp, and its crew of Filipinos and Nigerians

Both sides have issued claim and counter-claim about military victories, which are hard to verify due to travel restrictions in the Niger Delta.

The Nigerian military said it freed four Nigerian and nine Filipino hostages on Friday when it attacked a camp belonging to General Tompolo, the leader of the MEND faction who took the hostages.

However, MEND said that two hostages were killed in the crossfire and it was still holding two others.

"The bodies of the dead men will be handed over to the Red Cross," Gbomo said. "MEND deeply regrets the avoidable deaths."

The released hostages said that the two dead men were Filipino.

MEND added that General Tompolo had survived the attack and had relocated to another camp with his men.

Matthew Maguire, a British hostage who has been held since late last year, has also been moved to a camp in the Niger Delta, MEND said.

The militant group on Thursday declared "all-out war" against the government and demanded the oil companies evacuate the area by midnight Friday.

MEND and other groups operating in the Niger Delta say they are fighting for a better share of wealth from the oil-rich region for local residents, who say the oil industry has ruined their agriculture and fishing livelihoods.

However, the government said the rebels are criminal gangs intent on stealing oil.

Attacks on oil facilities and workers have cut oil production in Nigeria, one of the world's largest crude oil exporters, by around 20 per cent.

Expatriate workers are often kidnapped for ransom or for use as human shields. (dpa)