Maiduguri - Boko Haram militants retook the town of Marte in Borno state, which had been liberated in February, the state deputy governor said.
"It is sad as we have been made to understand that Marte is today completely fallen under the control of the insurgents, which to us is a very huge set back," Borno's deputy governor Zannah Umar Mustapha said.
The recapture of the town followed two strikes this week on state capital Maiduguri, which left at least 12 people dead.
Suspected militants also attacked the village of Kojiti, killing seven people in the Madagali local government of neighbouring Adamawa state. It had been cleared of the Islamist militant group in late February.
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Boko Haram took over a territory larger than Belgium last year, killing thousands of people and displacing some 1.5 million. The group's six-year insurgency is part of its bid to establish an Islamist state.
It took the combined efforts of Nigerian, Chadian and Nigerien troops to push them out of most of the areas, while Cameroon repelled border attacks.
The Sambisa forest reserve remains their last major stronghold.
"Our thinking was that every other place should have been blocked so that the insurgency would be curtailed to a restricted area," Mustapha said. "But that has not been the case because the insurgents have been fleeing to other communities."