Four Nigerian soldiers declared missing last week were killed by pirates in the restive oil-rich Niger Delta and their bodies have been recovered, an army spokesman said on Sunday.

"The ... attack was carried out by a gang of sea pirates led by Shedrack Itokofuwei, alias Mammy Water, who hails from Azagbene... (in) Bayelsa State," Lieutenant Colonel Timothy Antigha said.

The remains of the four attacked last Thursday along the Eweleso River have been retrieved, said Antigha.

Itokofuwei and his gang left their base in a convoy of five speed boats and on their way, attacked the soldiers.

In an ensuing encounter with a navy patrol, three of the pirate boats were sunk while two others escaped, he said in the statement.

Itokofuwei was among those killed "in the fire fight as disclosed by his gang member captured close to the scene of encounter," the statement added.

The soldiers killed, among them a lieutenant-colonel who was commander of a unit, were ambushed and attacked not far from where four marine policemen were shot dead at a checkpoint the same day.

A militant group in the region, the Movement for the Emancipation of the Niger Delta (MEND), on Friday claimed it was behind the killing of the policemen. It gave no motive for the attack.

MEND militants claim to have been fighting for a fairer distribution of oil revenue in Nigeria.

The Joint Task Force, deployed in the region to tackle crime, including oil theft and piracy, warned that it was prepared "to crush any obstacle on the path to lasting peace and security in the Niger Delta region."

Oil, the mainstay of Nigeria's economy, is derived from the Niger Delta, President Goodluck Jonathan's home region.

A 2009 government amnesty offer for the militants greatly reduced attacks in the region after more than 25,000 ex-fighters laid down their arms.

Most of MEND's commanders and their fighters took up the amnesty offer, but MEND spokesman Jomo Gbomo said the group was never part of the deal.