“We announce our allegiance to the Caliph of the Muslims, Ibrahim ibn Awad ibn Ibrahim al-Husseini al-Qurashi,” Boko Haram leader Abubakar Shekau said, referring to IS leader Abu Bakr al-Baghdadi.
Shekau said the pledge was made because of religious duty to Islam and because it would “enrage the enemy of Allah”.
The eight-minute speech, in which Shekau was not shown, was published on a Twitter account used by Boko Haram and subtitled in English, French and Arabic.
Shekau has previously mentioned al-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, has been seeking a closer tie-up.
Not only did Shekau announce last year that the captured town of Gwoza in Borno state was part of a caliphate but in recent weeks Boko Haram videos have increasingly resembled IS group propaganda.
Nigeria’s President Goodluck Jonathan, who has persistently blamed the violence on outside forces, last month claimed the country had intelligence on Boko Haram links to IS.
Sunni jihadist specialist Aaron Zelin, a senior fellow at the Washington Institute of Near East Policy, said it was difficult to assess the immediate effect of Shekau’s statement.
“Definitely it will put an even bigger target on their back,” he said, adding, “It’s relevant because it highlights the resonance of the idea of the caliphate.
“For years there have been rumours of connections with AQIM (Al-Qaeda in the Islamic Maghreb) or Al-Shebab but there was never anything definitive... and now (Boko Haram) decides to do this overtly.”
Max Abrahms, from Northeastern University in Boston, Massachusetts, said that with IS and Boko Haram both “uninhibited” in terms of violence, they were “a natural ally”.
But the apparently more formal link could attract more global attention to the insurgency.
“I think Shekau now has to be careful here as traditionally the international community has regarded the conflict as a civil war within Nigeria, perhaps an extended regional war in west Africa,” he added.
Boko Haram have been squeezed out of some captured territory, and security analysts have predicted a rise in bomb attacks in towns and cities, including to disrupt elections in three weeks’ time.
Yesterday, a woman with explosives strapped to her body blew herself up at Baga fish market in the Borno state capital, Maiduguri.
About an hour later another blast rocked the popular Monday Market, causing chaos as locals voiced anger at security forces who struggled to control the scene.
Just after 1:00 pm a third blast hit a used car lot which is attached to the busy Borno Express bus terminal.
Borno’s police commissioner Clement Adoda gave a toll of 58 dead “for the three locations” in Maiduguri and 139 wounded.