Nigerian armed forces said Friday 12 soldiers are missing following a week-long military offensive against rebels in the restive oil-rich Niger Delta.

Shortly, afterwards, the most active armed group in the region, the Movement for the Emancipation of the Niger Delta (MEND), claimed in a statement that its fighters had killed 11 soldiers in an ambush.

"We are still looking for 12 soldiers. We won't presume them dead until their bodies have been found," Joint Task Force (JTF) spokesman Colonel Rabe Abubakar told AFP.

He said the military were trying to locate the missing soldiers and to release the remaining hostages being held by the militants.

"This is why our cordon and search operation will continue," he said.

He said a notorious militant leader Government Ekpemupolo, alias Tompolo, was wanted in connection with "various acts of criminality in the region."

"Tompolo has to account for the missing soldiers, hostage-takings and oil thefts in the Niger Delta," he said.

MEND said that "11 more soldiers were ambushed and killed last night at Oporoza by a small commando unit. This operation will be a daily occurrence in the entire region."

It was not clear if the 11 soldiers were among the 12 declared missing by the military.

MEND also denied that government troops recovered arms and ammunitions from their abandoned camps.

"The fact is that none of the weapons displayed were recovered from our camps as claimed," it said.

"Our standard issue is the AK47 assault rifle and none was displayed. Instead rusty Dane guns used by hunters and old machetes and other weapons taken from God knows where were displayed," the group added in an email to the media.

Four Ukrainians seized from a fishing vessel have also been rescued, while two camps operated by the rebels have been destroyed.

MEND says it lost four fighters in the violence and accused the Nigerian military of bombing civilian targets.

The group which claims to be fighting for a greater share of Nigeria's oil wealth for local people, has been behind a series of kidnappings and attacks on oil installations in the past three years.

Unrest in the region has reduced Nigeria's daily output to 1.76 million barrels compared with 2.6 million barrels in January 2006.