Nigeria's military has announced the death of Abu Musab al-Barnawi, the head of the West African branch of the Islamic State group.

"He is dead and remains dead," said Chief of Defence Staff General Lucky Irabor.

Gen Irabor did not give any details of the circumstances of Barnawi's death, which was first reported in September.

The Islamic State West Africa Province (Iswap) has not commented on the claims.

Iswap has been seen as the strongest jihadist group in Nigeria since the death of Boko Haram leader Abubakar Shekau earlier this year.

Since then, thousands of Boko Haram fighters have surrendered both to the military and reportedly to Iswap.

Who was Barnawi?

Little is know about Barnawi, including his age and appearance.

Born Habib Yusuf, it is believed he was the eldest son of Boko Haram founder Mohammed Yusuf.

He was seen as a relatively moderate, shunning Boko Haram's more extreme policies like using children as suicide bombers, and the indiscriminate targeting of Muslims.

After his father's death in police custody in 2009, Shekau was appointed as the group's new leader.

Barnawi served as a spokesperson for Boko Haram, but frequently clashed with Shekau and other senior leaders and in 2013 he defected to Ansaru - a Boko Haram offshoot with ties to al-Qaeda.

Despite their differences, the two groups worked closely together at times.

To help raise Boko Haram's international profile, Shekau aligned the group with Islamic State (IS) in 2015. The following year IS named Barnawi as Boko Haram's new wali (Arabic for governor), causing a major internal feud. Analysts believe the change of leadership was prompted by ideological clashes between Shekau and IS' central leadership.

IS newspaper al-Nabaa published an interview with Barnawi in August 2016. In the article, he described the group's battle with West African states as one against "apostates" and "crusaders". He threatened, as leader, to order the killing of Christians and the bombing of churches. But in a major shift in strategy for the group, he pledged to end indiscriminate attacks on mosques and markets.

The high-profile change of leadership was not welcomed by everyone, and Shekau accused Barnawi of fomenting a coup.

As a result of this infighting, those loyal to Islamic State joined the breakaway Iswap, led by Barnawi, while Shekau stayed on as head of Boko Haram. The groups have since been staunch rivals.

Iswap announced that Shekau died in May after fleeing a battle with Iswap fighters - choosing to detonate a suicide vest instead of surrendering. Iswap said the operation, in Nigeria's Sambisa forest, was directly ordered by Islamic State's central leader Abu Ibrahim al-Hashimi al-Qurashi.

According to Nigerian outlet HumAngle, Barnawi shared news of his death in an audio recording in June, saying that Shekau had committed "unimaginable terrorism."

"When it was time, Allah set out brave soldiers after receiving orders from the leader of the believers," Barnawi reportedly said.

Later that month, alleged Boko Haram militants confirmed Shekau's death in a video published by Nigerian news outlets and security analysts.

IS has also confirmed the details of Shekau's death, and boasted that "thousands" of Boko Haram fighters have since defected.

Under Barnawi's leadership, Iswap made territorial gains in northern Nigeria, and the wider Chad Basin, during recent years. It is also active in neighbouring countries, including Burkina Faso, Cameroon, Chad, Niger and Mali.

The group has captured several military bases, securing weapons and supplies from regional military forces. Taxes on local residents have also provided it with a source of income, along with its involvement in commercial enterprises like fishing.