Nigerian militant group Boko Haram has pledged allegiance to Islamic State (IS), according to an audio statement.

The message, which could not immediately be verified, was posted on Boko Haram’s Twitter account and appeared to be by the group’s leader.

Boko Haram began a military campaign to impose Islamic rule in northern Nigeria in 2009. The conflict has since spread to neighbouring states.

It would be the latest in a series of groups to swear allegiance to IS