Unicef reckons 4.5-million people need assistance, one million in danger of extreme malnutrition.
Military spokesman says a number of militants were killed and two captured alive.
More than three-quarters of homes and other buildings in Bama, Borno’s second-largest town, have been destroyed by insurgents.
President Buhari has made crushing a six-year insurgency his priority, prompting intensive ground and air offensives against Boko Haram.
Cameroon deports over 12,000 Nigerians, and Chadian forces killed at least 117 Boko Haram fighters in recent weeks.
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Buhari left for Benin hours after at least six people were killed when a suicide bomber detonated explosives in the northeastern city of Maiduguri.
Nigerian army has led a series of raids against the jihadists in recent months, succeeding in freeing several dozen women and children.
The spike in violence has sparked concern that gains made by the armies of Nigeria, Niger, Chad and Cameroon since February are being eroded.
At least 55 people were killed last week in two raids on villages near the Borno state capital, Maiduguri -- the first attacks there in three months.
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Boko Haram's six-year insurgency has claimed 15,000 lives and displaced about 1.5 million people
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A mob formed threw stones at the security services, following complaints that they have repeatedly failed to contain the violence.
The militants have grown stronger, secured powerful new weapons and refreshed their ranks while the Nigerian military is mired in crisis.