In an audio message purportedly from an Islamic State spokesman, the group announced that a pledge of allegiance from Nigerian-based Boko Haram has been accepted by IS leader Abu Bakr al-Baghdadi.
The 28-minute message reported by the SITE intelligence monitoring service, a Washington group which tracks statements and videos released by extremists, cannot be independently verified by Channel 4 News.
The IS group militant Abu Mohammed al-Adnani, who claims to be a spokesman, was responding to an audio recording posted online by Boko Haram leader Abubakar Sheka on Saturday that pledged allegiance to the IS.
He said: “We announce to you to the good news of the expansion of the caliphate to west Africa because the caliph… has accepted the allegiance of our brothers of the Sunni group for preaching and the jihad.”
The spokesman also urges Muslims to join militants in west Africa, rejecting suggestions that Iraqi forces and the US-led coalition have recently had a series of victories against IS in Iraq and Syria.
Boko Haram has been weakened by a multinational force that has dislodged it from a score of northeastern Nigerian towns.
Founded in 2002, initially focused on opposing western-style education – Boko Haram means “Western education is forbidden” in the Hausa language.