By Tife Owolabi

YENAGOA, Nigeria (Reuters) - Nigerian militants have ambushed a police boat in the oil-producing Niger Delta region, killing 12 police officers, authorities said on Sunday.

The Movement for the Emancipation of the Niger Delta (MEND), the main delta militant group before a 2009 amnesty, claimed responsibility for Friday's attack, staged shortly after it had threatened to resume its campaign of violence.

However, another of the armed groups roaming Africa's top oil producer said it had carried out the ambush near the village of Azuzama, which a further 38 police officers survived.

MEND, whose activities at their peak cut oil production in Nigeria by around half before the amnesty, threatened to restart attacks this week in retaliation for the jailing of its leader Henry Okah by a South African court.

MEND said it had attacked the boat heading to a funeral in Bayelsa state because government forces had refused to take its threat seriously.

"For dismissing (our statement) ... as an 'empty threat', heavily armed fighters from ... MEND intercepted and engaged government security forces in a fierce gunfight at Azuzama," said an emailed statement signed by Jomo Gbomo, a pseudonym the group uses.

"All oil companies and the public are advised to ignore the false sense of security being peddled," it said, maintaining that the attack had been on Saturday, rather than Friday.

Bayelsa state governor Henry Dickson confirmed the 12 policemen were dead, calling the incident "most tragic, shocking and disheartening".

A resurgence of militant activity would be a blow to President Goodluck Jonathan, who helped to negotiate the amnesty and who is from the same Ijaw ethnic group as most of the militants. His administration's security forces are already stretched by an Islamist insurgency in the north.

It would also be a major problem for multinational energy companies such as leading operator Royal Dutch Shell, already contending with industrial scale oil theft by armed gangs that saps up to a fifth of Nigeria's 2 million barrel-a-day output, according to some government estimates.

Police Commissioner Kingsley Omire said the ambush was actually carried out by militants once loyal to Kile Selky Torughedi, who headed MEND's southern wing.

He blamed a dispute between the gunmen and Torughedi over government payments made under the amnesty deal, which aims to achieve peace in the delta by paying off thousands of militants.

LOCAL DISPUTE?

A militant from the faction, Jaspa Adaka, told Reuters it had attacked the boat, believing wrongly that Torughedi was aboard and heading to the funeral with police protection.

Adaka said it was a "warning" to Torughedi, whom he accused of embezzling amnesty funds meant for the faction's foot soldiers.

The government is keen to wind the amnesty payments down but fears the militancy will restart as soon as it does so.

Omire said the boat carrying 50 police officials was heading to the funeral late on Friday when it developed engine problems in one of the winding creeks of the swampy delta region that is home to Africa's biggest oil industry.

"The craft developed an engine problem ... and the officers became soft targets for some hoodlums, who we have confirmed were part of a militant group that was supposed to be enjoying an amnesty," Omire said, adding that all others on the boat were safe.

While attacks in the Niger Delta region have dropped since the amnesty, kidnapping, piracy, large-scale oil theft and pipeline sabotage still occur almost daily.

MEND leader Okah was sentenced to 24 years in prison on March 26 for masterminding two car bombings in the Nigerian capital of Abuja in 2010 that killed at least 10 people.

MEND has been largely inactive since most of its members agreed to the amnesty, although the group claimed an attack on crude oil pipeline owned by Italian oil and gas group Eni in April last year.

As well as having a leader in jail, most of the group's senior commanders are enjoying generous government amnesty payouts, and many of them no longer live in the delta.

(Writing by Tim Cocks; Editing by David Stamp)