Islamist insurgents overran a Nigerian army base in the north-east, killing at least the commander, two Nigerian security sources said.
Militants took a Nigerian army base at Kareto in north-eastern Borno state security sources said.
No other details about further casualties were available.
Nigerian army spokespeople did not immediately respond to requests for comment.
The latest development is at odds with the Nigerian military’s public statements as recently as Monday that campaigns against Boko Haram and Islamic State’s West African branch are making “major breakthroughs”, statements that frequently describe the insurgents as “remnants”.
It was not immediately clear whether the attackers were affiliated with Boko Haram or Islamic State.
The insurgents were said to have overrun troops from the Nigeria Army’s 158th Battalion stationed at Kareto, killing at least their commanding officer and causing soldiers to flee, one security source said.
After the assault, the militants left Kareto but remained in the area, they said.
President Muhammadu Buhari in his second-term inaugural speech on Wednesday said Nigeria was meeting security challenges “with greater support to security forces in terms of money, equipment and improved local intelligence”.
Nigeria’s government said the Boko Haram insurgency and the rival Islamic State West Africa Province group are on their last legs.
Sustained efforts to eradicate the militants failed and the military continues to suffer heavy losses.
The insurgency has killed over 30,000 and displaced millions in affected areas.