Nigerian Islamist group Boko Haram have captured yet another town and released an Isis-style video in which the militants appear on an armoured tank patrolling the streets preaching Islam to locals.

The vehicle was parading down a road in the unidentified town, which was seized by the Islamists.

In the 44-minute video obtained by the AFP, the extremist group's leader Abubakar Shekau also denied reaching any agreement with the Nigerian army for releasing the kidnapped schoolgirls.

The veracity of the footage remains questionable as it is not known whether it was propaganda material staged by the insurgent group. Suspicions about the video have risen since the local residents are seen cheering the jihadist fighters.

The militant leader also pledged support to other Islamic Caliphates across the world including the Sunni militant group Islamic State, which is currently terrorising Iraq and Syria.

Shekau's Islamic preaching is broadcast onto the streets via loud-speakers in the video as the vehicle rolls on the neatly-paved road.

"We have indeed established an Islamic caliphate," Shekau says in the video.

"There is no truce between me and Nigerian tyrants [Army]. This person [supposed rebel negotiator Danladi Amhadu]... if he thinks he can, let him show his face. By Allah, we will kill him."

The video has also come at a time when a deadly explosion rocked a school assembly in Yobe killing dozens of students.