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's extremist group has confirmed the death of its chief Abubakar Shekau, who sources say died during infighting with a rival -allied faction, according to a video message from its presumed new commander.

In the short video in Arabic, top Boko Haram commander Bakura Modu, also known as Sahaba, urged his faction's commanders to remain loyal despite the loss of their historic commander.

Shekau's death was seen a major shift in Nigeria's conflict, though Islamic State in West Africa Province or ISWAP has recently emerged as the dominant force in the more than decade-long militant insurgency in the country's northeast.

The video, provided to AFP by a source close to Boko Haram and confirmed to be Bakura Modu by another local source, illustrates that militant infighting is far from over in Africa's most populous nation.

Shekau, who gained notoriety after kidnapping nearly 300 schoolgirls in 2014, killed himself last month rather than surrender after IS-allied rivals attacked his base camp in Nigeria's northeast Borno state, security sources said.

In an audio, ISWAP commander Abu Musab Al-Barnawi had already claimed Shekau killed himself while on the run from ISWAP fighters.

The undated Boko Haram video shows Bakura flanked by scores of armed fighters in formation as he addresses the camera, in what is traditionally a militant group's way of presenting a new leader.

"Commanders of Jama'atu Ahlu Sunna Lidda'awati Wal Jihad. Be steadfast and draw your swords, spreading your jihad, decapitating the enemy," he said, using the official name of Shekau's Boko Haram faction also known as JAS.

"Don't allow what befalls you these days to weaken your resolve on the jihad your are waging, because Allah has not forsaken your efforts."

He also urges Boko Haram commanders to reject ISWAP commander Barnawi.

Soon after the initial reports of Shekau's death last month, infighting between Nigeria's two rival extremist factions intensified. ISWAP fighters moved against Boko Haram commanders who refused to surrender and join their ranks, intelligence sources have said.

The two factions have skirmished in the past since ISWAP split from Boko Haram in 2016, objecting to Shekau's indiscriminate targeting of Muslim civilians and use of women suicide bombers.

Bakura's faction is one of the Boko Haram elements operating in the Lake Chad area where they have access to porous borders with Chad and Niger.

ISWAP fighters have taken over Shekau's stronghold in the Sambisa forest in their bid for consolidation, analysts and security sources have said.

But they said Barnawi would face potential resistance from hardline, pro-Shekau factions along Nigeria's border regions.

More than 40,000 people have been killed and over two million displaced from their homes in fighting since the conflict in northeastern Nigeria began in 2009.