Nigeria's President Muhammadu Buhari has given his new military chiefs a three-month deadline to defeat the Islamist militant Boko Haram group, according to the BBC.

He gave the order at a swearing-in ceremony for the new service commanders he appointed last month.

When Buhari took office in May, he vowed to tackle the six-year Islamist insurgency "head on".

He has made a boosted multinational force of 8,700 central to his strategy in tackling the crisis.

At least 17,000 people have been killed since Boko Haram launched its insurgency in northern Nigeria 2009, according to Amnesty International.

Although the militants have lost their strongholds, they are still active and there has been an upsurge in suicide attacks since Buhari took office.