The notorious Islamic State terrorist organisation has formally accepted the offer of membership from Nigeria’s terrorist group, Boko Haram. The acceptance is contained in an audio recording released yesterday presented as a speech by top IS spokesman Abu Mohammed al-Adnani.

Boko Haram had pledged allegiance to IS on Saturday but the move had yet to be formally accepted by IS supremo Abu Bakr al-Baghdadi for the Nigerian extremist group to become part of the “caliphate”.

“We announce to you to the good news of the expansion of the caliphate to West Africa,” Adnani said, according to an AFP despatch.

“Our caliphate is resisting and it is advancing in the right direction. We are fighting the Crusaders and the rafidah (Shiites) and day by day the Islamic State is becoming strong,” he said.

He insisted that the group is “sure of its victory” regardless of the challenges.

“God is on our side and give us the strength to combat this armada of Crusader countries,” he said.

•Photo shows IS supremo Abu Bakr al-Baghdadi.