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Boko Haram has pledged allegiance to the Islamic State group in an audio message published online, after three bombs wreaked havoc in northeast Nigeria, killing 58 and wounding scores.
"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.
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.
