"We announce our allegiance to the Caliph of the Muslims, Ibrahim ibn Awad ibn Ibrahim al-Husseini al-Qurashi," said the voice on the message, which was believed to be that of Shekau and was released through Boko Haram's Twitter account.
Qurashi is better known as Abu Bakr al-Baghdadi, leader of the IS group which has proclaimed a caliphate in parts of Syria and Iraq.