(BBC News) Nigerian militant group Boko Haram has pledged allegiance to Islamic State (IS), according to an audio statement.
The message, which has not been verified, was posted on Boko Haram's Twitter account and is believed to be by the group's leader, Abubakar Shekau.
Boko Haram began a military campaign to impose Islamic rule in northern Nigeria in 2009. The conflict has since spread to neighbouring states.
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It would be the latest in a series of groups to swear allegiance to IS.