From the BBC:
BBC News - Boko Haram declares 'Islamic state' in northern NigeriaMilitant group Boko Haram has said it has set up an Islamic state in the towns and villages it has seized in north-eastern Nigeria...
Nigeria's army has rejected the claim as "empty".
Boko Haram essentially formalized what had been a de facto situation. Given its battlefield position, Nigeria's army is not in any position to declare the declaration "empty." It remains very unlikely that the army will mount any meaningful offensive against the Boko Haram terrorist organization, much less a successful one, to drive it from northeastern Nigeria.