Armed men have ambushed a police boat in Nigeria's oil-producing Niger Delta region, leaving 12 officers missing and presumed dead, security officials said.
Police Commissioner Kingsley Omire said on Sunday the boat, carrying 50 police officials, was heading to a funeral late on Friday.
The vessel developed engine problems in one of the winding creeks of the swampy delta region that is home to Africa's biggest oil industry, he said.
"The craft developed engine problem was now isolated and the officers became soft target for some hoodlums, who we have confirmed were part of a militant group that was supposed to be enjoying an amnesty," Omire said.
A Nigerian security source, who could not be named because he was not authorized to speak to the media, said the 12 were very likely all dead, although Omire said all others on the boat were safe.
Sunday's ambush came days after the Movement for the Emancipation of the Niger Delta (MEND), the main group which has been fighting for a greater share of the country's oil wealth, a threatened to restart attacks in retaliation for the jailing of leader Henry Okah by a South African court.
The group had agreed in 2009 to halt attacks in the delta after the government promised to look into its grievances.
Omire said the ambush was carried out by gunmen once loyal to Kile Selky Torughedi, who headed MEND's southern wing.
But he denied there was a link to the threat by the group, whose attacks cut oil production in Nigeria by around half before the amnesty.
He instead blamed a dispute between the gunmen and the government over their amnesty payments - underscoring the fragility of the peace that has been achieved in the delta by paying off thousands of fighters to silence their guns.
The government is keen to end amnesty payments but fears the move could trigger renewed attacks.
Any new attacks would be a blow to President Goodluck Jonathan, who helped negotiate the amnesty and who is from the same Ijaw ethnic group as most of the armed men.
His administration's security forces are already stretched by armed groups in the north.