Lagos - A statement on Sunday purporting to be from the main militant group in the country's oil-producing region, MEND, claimed an attack on security forces, alleging at least 15 officers were killed.
It was however not possible to determine if the statement was authentic. Police have previously claimed that recent violence in the Niger Delta was not linked to the group, which has been mostly inactive in the years since a 2009 amnesty deal.
The statement spoke of an attack on Saturday, while authorities have said a police boat was ambushed on Friday in the same area.
"Heavily armed fighters from the Movement for the Emancipation of the Niger Delta (Mend), at about 17:00 hours, Saturday 6 April 2013, intercepted and engaged government security forces in a fierce gunfight lasting over 40 minutes at Azuzama, Southern Ijaw, Bayelsa State, Nigeria," the statement sent by e-mail to journalists 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."
Authorities meanwhile have said that attackers ambushed and opened fire on a police boat on Friday in Bayelsa state in the Niger Delta region of southern Nigeria, leaving 12 police officers missing.
Bayelsa Governor Henry Dickson issued a statement on Sunday confirming the 12 policemen were killed.
The attack came after another statement last week, again purporting to be from Mend, which threatened to resume attacks over the conviction of the organisation's alleged leader Henry Okah in South Africa.
Police however said there was no link and the ambush appeared to be related to a dispute between an ex-militant and his gang.
Authorities said the attack in the swamps of Bayelsa came while officers were escorting the ex-militant for his mother's burial.
According to police, the attackers were members of the gang linked to the ex-militant and who have alleged that he had not properly distributed amnesty payments.
Mend has broken down in the years since the 2009 amnesty, with most major militant leaders having embraced the deal.
However, Nigeria can ill afford a return to violence in the Niger Delta, with security forces already battling a deadly insurgency by Islamist extremist group Boko Haram in the country's northern and central regions.
Nigeria derives some 80% of its revenue and almost all of its export earnings from the oil industry.