Nigeria's main militant group has declared "all-out war" in the heart of Africa's biggest oil and gas industry after the country's security forces used gunboats against fighters.

Navy gunboats exchanged fire with militants along Chanomi Creek in Delta state, a region which is home to US energy giant Chevron's export terminal and a 125,000-barrels-per-day refinery.

The military says it carried out the operation after the hijacking of two oil vessels, attacks on soldiers and repeated warnings to oil firms to evacuate their staff.

Eighteen Filipinos and four Nigerians have been kidnapped from one of the hijacked vessels.

"The military task force cannot just fold its hands and allow these sorts of barbaric events to continue," military spokesman Colonel Rabe Abubakar said

The Movement for the Emancipation of the Niger Delta (MEND) says one Filipino national was killed by "stray bullets from the Nigerian army" during clashes with the fighters.

Colonel Rabe Abubakar denied the rebel account, saying "It is a lie. None of them were killed."

MEND said the military had used aircraft during the attack in the Gbaramatu area of Delta state and that women, children and the elderly were among the casualties, a charge Colonel Abubakar also denied.

"MEND is declaring an all-out war in the region and call upon all men of fighting age to enlist for our freedom," the group said in a statement emailed to media, repeating an ultimatum for oil workers to leave the region by midnight.

MEND has issued such threats several times in the past, most recently in late January when it warned of a "sweeping assault" on the oil and gas industry which never materialised.

Security sources have said they are taking the militant threats seriously but there are no plans to evacuate more staff.

"We are keeping a low profile and hunkering down," one oil industry source said, adding operations were continuing.

Threat to industry

Violence in the delta has cut Nigeria's oil output by about a fifth since early 2006, forced foreign firms to remove all but essential employees and eaten into the OPEC member's earnings, exacerbating the impact of the global downturn.

Friday's unrest is the most serious since militants attacked industry sites in what they dubbed a six-day "oil war" eight months ago.

The unrest offered some support to oil prices, which were down around $2 at $56.61 a barrel on Friday.

ABC/AFP/Reuters