The Islamic State group has accepted a pledge of allegiance to the group made by the Nigerian jihadist organisation Boko Haram, according to an audiotape from its spokesman.

"We announce to you to the good news of the expansion of the caliphate to West Africa because the caliphate ... has accepted the allegiance of our brothers of the Sunni group for preaching and the jihad," IS spokesman Mohammed al-Adnani said in the message Thursday, using the name in Arabic of the Nigerian group.

Itself a radical Sunni Muslim movement, IS has seized large swathes of Iraq and Syria and declared an Islamic "caliphate" there, and has also drawn expressions of allegiance from jihadists in Egypt and Libya.