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Islamic State (IS) has accepted a pledge of allegiance from Nigeria's militant group Boko Haram, according to an audio message, the BBC reported.

In the tape, which has not been verified, an IS spokesman says the aim of establishing a caliphate has now been expanded to West Africa.

Last week, Boko Haram posted a message saying it wanted to join ranks with IS.

Boko Haram began a military campaign to impose Islamist rule in northern Nigeria in 2009.

The conflict has since spread to neighbouring states.

In the tape, a man - who describes himself as IS spokesman Mohammed al-Adnani - says: "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.

The audio tape has not been independently verified.