AFP Report: IS welcomes Boko Haram allegiance.

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

  "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," IS spokesman Mohammed al-Adnani said in the message, using the Arabic name for Boko Haram.

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.

On Saturday, an audiotape attributed to Boko Haram leader Abubakar Shekau said "we announce our allegiance to the Caliph of the Muslims, Ibrahim ibn Awad ibn Ibrahim al-Husseini al-Qurashi," referring to IS leader Abu Bakr al-Baghdadi.

Shekau has previously mentioned Baghdadi in video messages yet stopped short of pledging formal allegiance.

But there have been increasing signs that the Nigerian militants, whose six-year insurgency has claimed more than 13,000 lives and left 1.5 million people homeless, have been seeking a closer tie-up.

IS spokesman Adnani urged Muslims to join militants in West Africa and insisted that the caliphate was growing.

"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 jihadist group is "sure of its victory" regardless of the challenges it is facing.

For months, IS has been targeted with air strikes from a coalition led by the United States and suffered territorial setbacks in Syria and Iraq.

And Iraqi government forces have closed on the city of Tikrit this week in a bid to retake it from the group.