LAGOS, Nigeria—Nigeria’s military claimed Thursday that Abu Musab al-Barnawi, the leader of an ISIS-linked extremist group blamed for killing hundreds in the northeast, had died. There was no immediate confirmation from the militants. At a news conference, Nigeria’s chief of defense staff, Gen. Lucky Irabor, told reporters: “I can authoritatively confirm to you that Abu Musab is dead.” He gave no further information and it was not possible to independently corroborate the claim. The announcement came only five months after al-Barnawi and his forces claimed responsibility for killing rival extremist leader Abubakar Shekau. Some reports said al-Barnawi had been fatally wounded during clashes with yet another rival extremist faction, but the military gave no details about how it had confirmed his death. Al-Barnawi was just a teenager when his father, Mohammed Yusuf, was killed by Nigerian security forces in 2009. The death of the founding Boko Haram leader has fueled …