The development comes as both terror groups face increasing military pressure and have suffered setbacks on the battlefield.
IS seized much of northern and western Iraq last summer, gaining control of about a third of both Iraq and Syria. But it is now struggling against Iraqi forces seeking to recapture Saddam Hussein’s home town of Tikrit, while coming under fire from US-led coalition air strikes in other parts of the country and in Syria.
Boko Haram, meanwhile, has been weakened by a multinational force that has dislodged it from a score of north-eastern Nigerian towns. But its new Twitter account, increasingly slick and more frequent video messages, and a new media arm were all considered signs that the group is now being helped by IS propagandists.
Last Saturday, Boko Haram leader Abubakar Shekau posted an audio recording online which pledged allegiance to IS. On Thursday, IS’s media arm, Al-Furqan, in an audio recording by spokesman Abu Mohammed al-Adnani, said Boko Haram’s pledge of allegiance has been accepted, claiming the caliphate has now expanded to West Africa.
Al-Adnani had urged foreign fighters from around the world to migrate and join Boko Haram.
J Peter Pham, director of the Africa Centre at the Atlantic Council, a think-tank in Washington, said the alliance highlights a new risk.
“Militants finding it increasingly harder to get to Syria and Iraq may choose instead to go to north-eastern Nigeria and internationalise that conflict,” he said.
In the past – as was the case with IS affiliates in Egypt, Yemen and Libya – it took weeks for IS to respond to a pledge of allegiance.
“The prompt – one might even say fast-tracked – acceptance by the so-called Islamic State of Boko Haram’s pledge of allegiance” underscored that both needed the propaganda boost from the affiliation, Mr Pham added.
Boko Haram’s pledge comes as the militants were reported to be massing in the town of Gwoza for a showdown with the Chadian-led multinational force.
Boko Haram killed an estimated 10,000 people last year, and has been blamed for last April’s abduction of more than 275 schoolgirls. For six years the group has been waging an insurgency to impose Islamic or Sharia law in Nigeria.
Boko Haram followed the lead of IS in August by declaring an Islamic caliphate in north-east Nigeria which grew to cover an area the size of Belgium. After their blitz last year, IS extremists declared a caliphate in the swathes of territory they control in Iraq and Syria and imposed their harsh interpretation of Islamic law.
The Nigerian group has also followed IS in publishing videos of beheadings.
On Thursday members of the UN Security Council proposed that the international community supply money, equipment, troops and intelligence to a five-nation African force fighting Boko Haram.