The video fuels fears that ISWAP is consolidating control of the insurgency in northeastern Nigeria following the death of Boko Haram leader Abubakar Shekau last month. But not all observers are convinced the video is proof that all Boko Haram fighters are ready to join ISWAP.
The groups have been engaged in a violent rivalry for years and if ISWAP absorbs Boko Haram fighters it could focus attention on attacking the Nigerian military.
Some 350,000 people have died as a result of the 12-year insurgency and subsequent humanitarian crisis, the United Nations said this week.
The video, produced by Islamic State's official media arm, showed clips of several hundred men, many of whom were armed, gathering in the bush.