Nigeria's main militant group said on Sunday they killed more than 15 security forces during a fierce gunfight at Azuzama area of southern Bayelsa State the previous day.

An unknown number of policemen were reported missing in Bayelsa State after an attack by militants on a team of police officers in the creeks.

The ambush in the village of Azuzama was the most serious attack in months in the region and came against the backdrop of threat by the Movement for the Emancipation of Niger Delta (MEND) to resume hostilities to protest the recent conviction of Henry Okah for terrorism by a South African court.

In a statement to the media, MEND spokesman Jomo Gbomo said it carried out the attack because government forces dismissed its warning to resume hostilities in the Niger Delta region, following the jail sentence of their former leader Henry Okah in South Africa as an "empty threat."

"For dismissing Hurricane Exodus as an 'empty threat' by the Nigerian security forces, heavily armed fighters from the Movement for the Emancipation of the Niger Delta (M.E.N.D), at about 17:00 Hrs. (5:00 p.m. local time), Saturday 06, April 2013, intercepted and engaged government security forces in a fierce gunfight lasting over forty (40) minutes at Azuzama, Southern Ijaw, Bayelsa State, Nigeria," the group spokesperson said.

"The clash which happened in the river left over 15 security forces dead as we also lost two of our fighters in the battle," the group added.

"We hope this encounter will serve as a lesson to the Joint Task Force (JTF) from making careless utterances that cannot be backed as we remain resolute in our resumption of hostilities," it declared.

On Wednesday, the MEND group threatened to resume new hostilities in the oil-rich Niger Delta region following the conviction of Okah in South Africa.

The militant group said a fake letter was used as evidence against Okah for which a 13-year sentence was passed.

The militant group reiterated that it will launch a major operation to simultaneously affect oil facilities across the Niger Delta region in the coming days.

The group held responsible for a series of attacks on international oil companies in the region is linked to sabotage, theft, property destruction, guerilla warfare and kidnapping.

Until recently, the Niger Delta region remains unstable with clashes frequently reported.

More than 400 foreigners have been seized in the region since 2006. Almost all have been released unharmed after a ransom was paid.