Abuja (AFP) - Nigeria's military on Wednesday claimed for the first time that Boko Haram leader Abubakar Shekau was dead, as it said troops had killed a lookalike who had been posing as the militant commander.

Defence spokesman Chris Olukolade told reporters in Abuja that a heavily bearded Islamist fighter identified as Mohammed Bashir died during fighting in the town of Konduga, in Borno state.

Bashir, who was said to have had several aliases, had "been acting or posing on videos as the deceased Abubakar Shekau, the eccentric character known as leader of the group", he added.

The announcement is the first time the military has said publicly that Shekau was dead after two previous claims by security sources that he had died in July 2009 and in late June 2013.

The military did not, however, say how or when Shekau died.

Earlier this year, the spokeswoman for the country's secret police, Marilyn Ogar, said "the original Shekau is dead" and that the person appearing in numerous videos was an imposter.

Olukolade said on Wednesday that the actual identity of Boko Haram's leader was not relevant. The name "Shekau" had become a "brand name for the terrorists", he told a news conference.

"The Nigerian military remains resolute to serve justice to anyone who assumes that designation or title, as well as all the terrorists that seek to violate the freedom and territory of Nigeria," he added.

The United States last year put up a $7 million reward for Shekau's capture as part of its Rewards for Justice programme and designated him a "global terrorist".

There has long been speculation in Nigeria and beyond about whether he was actually still alive.

It has been claimed that he actually died in 2003 and his name has since been used by at least two others.

They include Boko Haram members called Abdullahi Damasak, who was succeeded on his death by a Mustapha Chad, according to sources close to the group.

Olukolade told the briefing that Bashir had several identities: "Bashir Mohammed, alias Abubakar Shekau, alias Abacha Abdullahi Geidam, alias Damasak, etc."

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- Continued scepticism -

Whether the announcement will end speculation about Shekau's true identity or death is unclear and there was no independent verification of the claims.

The military showed footage of an amateur video recording of the fighting in Konduga, in which bodies littered the streets.

"That character tallies with the one that has been showing himself on the video," said Olukolade, pointing to a bearded man lying dead on the ground alongside another slain fighter.

A close-up still photograph of the man's face was also shown alongside a screengrab from a Boko Haram video of Shekau holding an assault rifle.

An arrow pointed to a small growth on the forehead of both men.

Analysts said earlier this week that they were sceptical about claims that he had been killed, as the same photograph shown by the military circulated online.

Ryan Cummings, chief Africa analyst at Red24 risk consultants in South Africa, said he thought it unlikely that Boko Haram's commander would be in the thick of battle in Konduga or anywhere else.

But Jacob Zenn, from the Jamestown Foundation think-tank in the United States, said the death of a body double in Konduga was plausible.

"It's important to note, however, that Shekau may have had 'doubles' who appeared in some videos. And the army has a record of being incorrect about claims of Shekau's death," he said.

Wait and see if another video emerges "from Shekau -- or someone who purports to be Shekau", he said.

Nigeria's military has been under pressure to regain territory lost to the Islamists in the far northeast in recent weeks, and has been trying to push back against the extremists.

Addressing the annual UN General Assembly in New York on Wednesday, Nigerian President Goodluck Jonathan vowed to step up the fight against Boko Haram, without mentioning Shekau.

"Let me underline today that we shall not waver until we end this mindless war on innocents and bring all the perpetrators to justice. We will triumph over terrorism," he said.