The spokesman for Islamic State militants said the extremist group has accepted the pledge of allegiance by West Africa's Boko Haram group.

In an audio recording released by the group's media arm Al-Furqan Thursday, Abu Mohammad al-Adnani says said the pledge of allegiance meant that the caliphate has now spread to West Africa.

The recording was released a few days after an audio recording from Boko Haram leader Abubakar Sheka posted online Saturday in which he pledged allegiance to IS.

Boko Haram in August followed the lead of IS in declaring an Islamic caliphate in northeast Nigeria that grew to cover an area the size of Belgium. The Islamic State had declared a caliphate in vast swaths of territory that it controls in Iraq and Syria.