A humanitarian source who requested anonymity said the sudden closures could be linked to negotiations to free aid workers who have been held hostage by the jihadist group Islamic State West Africa Province since July.

Late Wednesday, two army trucks stormed the offices of Action Against Hunger (ACF) in the city of Maiduguri, the capital of Borno State.

"They came and asked everyone in the building to leave. They said it was an order from above," a staff member said on condition of anonymity.

The aid worker said soldiers backed by two heavily armoured vehicles also shut down the group's office in the city of Damaturu in neighbouring Yobe state.

"The soldiers didn't explain why they shut the office. They only asked us to leave and not to take anything with us," another aid worker said, adding: "We don't know what is happening."

Last July, six ACF employees were abducted by jihadists from the Islamic State West Africa Province (ISWAP) -- a faction of Boko Haram affiliated with the Islamic State group -- as they were on their way back to Damaturu.

They remain in captivity.

- 'Without notice, without explanation' -

In a statement on Wednesday, ACF condemned the closures of their offices in Maiduguri and Damaturu.

"This decision without notice and without any explanation jeapordises the assistance ACF is providing to the most vulnerable in Borno State, and halts... the assistance ACF is providing to millions people," the agency said.

At a news conference in Maiduguri on Thursday, the army made no mention of the incident and declined requests for comment.

Relations between the Nigerian army and aid organisations in the region have been strained for years.

In August 2017, soldiers conducted a search of a UN camp in Maiduguri, denounced by Amnesty International.

According to Yan St-Pierre, a counter-terrorism consultant for MOSECON (Modern Security Consulting Group), there is a "sense of impunity" within the Nigerian military.

"The Nigerian army has accustomed us to act in a totally unpredictable way, but the violence of this closure is a bit different from what we are used to," the expert said.

"The military may believe that ACF is not revealing everything, or authorities suspect a mole within the NGO," said Yan St Pierre, a specialist in the conflict.

Despite claims by the Nigerian government that Boko Haram has been technically defeated, the insurgency continues to rage, devastating the region.

More than 35,000 have died since the conflict began, with some two million people displaced and many reliant on food aid.