The Islamic State West African Province (ISWAP) militant group said in an audio recording heard by Reuters on Sunday that Abubakar Shekau, leader of rival Nigerian militant Islamist group Boko Haram, was dead.

Shekau died around May 18 after detonating an explosive device when he was pursued by ISWAP fighters following a battle, a person purporting to be ISWAP leader Abu Musab al-Barnawi said on the audio recording.

"Abubakar Shekau, God has judged him by sending him to heaven," he can be heard saying.

Two people familiar with al-Barnawi told...