Militants kill15 soldiers and four militia fighters after attacking their convoy in northeastern Borno state, security sources say. Meanwhile, search continues for dozens of students kidnapped in northwestern Kaduna state.
A spokesman for UN chief Antonio Guterres confirmed only that there was a "security incident", but gave no further details.
Maiduguri city's three million residents plunged into darkness after a Daesh-linked militant group blew up transmission lines, in the third such attack in a month.
Meanwhile, Boko Haram has claimed responsibility for the abduction of the students – more than 300 boys – from northern Katsina State.
Nigerian security sources confirm that the attack by suspected militants on the security convoy killed policemen, soldiers, government-backed militia members and civilians.
They have seized those who had returned to the town in a government operation on August 2 after spending nearly two years in displacement camps.
The blast occurred at Kalam in the Lake Chad region, which has been battered by attacks from militants crossing from neighbouring Nigeria.
Mercy Corps says it's suspending operations in Borno and Yobe states after Nigerian military shut four of its field offices. Separately, Daesh-linked militants executed one of six captive aid workers of Action Against Hunger.
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