Nigeria's security forces have raided a UN camp in the north-eastern city of Maiduguri and carried out an unauthorised search, a UN official has told the BBC.
Samantha Newport, of the UN Office for the Co-ordination of Humanitarian Affairs (OCHA), says the search lasted nearly three hours.
The UN is urgently seeking answers from the authorities.
The base provides aid to those affected by the Boko Haram insurgency.
The BBC's Martin Patience in Lagos says one possible reason for the search could be the camp's name - Red Roof.
Rumours have been swirling in Maiduguri that the leader of Boko Haram, Abubakar Shekau, has been hiding out in a compound with the same name.
Edward Kallon, the Humanitarian Coordinator for Nigeria has expressed grave concern following the search.
Mr Kallon says that he is extremely concerned that the actions by the security forces could be "detrimental to the critical work that is being carried out every day to support the most vulnerable in the region".
The UN ordered local staff to work from home today following the incident.
It also said it grounded helicopters - which provide humanitarian assistance to far - flung camps - as a precautionary measure.