Meanwhile, in Nigeria, Boko Haram is carving out its own 'caliphate'

Boko Haram, whose name translates as "Western education is sin", is on the brink of ending government control across an entire Nigerian state, effectively establishing its own caliphate.

With the Islamic State offering advice on strategy and tactics, the worst-case scenario of once-symbolic links between the Islamist groups becoming a practical relationship is being discussed by Western intelligence agencies.

Boko Haram has apparently been emboldened by the gains of Isis in Iraq and Syria and is now behaving like a conventional army in the states of Borno, as well as Adamawa and Yobe, all in eastern Nigeria near the border with Cameroon and Chad.

The terror group caused global outrage in April when it kidnapped 300 schoolgirls from the village of Chibok, 200 of whom remain missing.

More: Who beheads more people: Isis or the government of Saudi Arabia?