MAIDUGURI, Nigeria (Reuters) - Nigeria's military raided a United Nations compound on Friday in the northeastern city at the epicenter of a conflict with Islamist insurgency Boko Haram, a UN official said.

The objective of the search could not be determined but it could damage an already tense relationship between the military, the United Nations and aid groups tackling one of the world's largest humanitarian crises sparked by the Boko Haram conflict.

"Members of the Nigerian security forces entered a United Nations base for humanitarian workers in Maiduguri ... without authorization," said Samantha Newport, a spokeswoman for the UN Office for the Coordination of Humanitarian Affairs.

The security forces, who arrived at about 5 a.m., "carried out a search of the camp and left at about 0800 hours," she said, adding that the UN has no information on the reason for the unauthorized search.

"The United Nations is extremely concerned that these actions could be detrimental to the delivery of lifesaving aid to the millions of vulnerable people in the northeast of Nigeria," Newport said.

The UN's top representative in Nigeria, the humanitarian coordinator, is communicating with the government about the incident, said Newport.

Nigeria's theater commander for the conflict with Boko Haram, Ibrahim Attahiru, told Reuters he did not know the reason for the raid on the UN compound.

Spokesmen for Nigeria's military did not respond to calls and messages seeking comment. A spokesman for Nigeria's presidency did not respond to a request for comment.

The eight-year insurgency has driven at least 2 million people from their homes and almost 7 million need humanitarian assistance. Tens of thousands live on the brink of famine and millions more lack secure access to food.

More than $650 million has been given by the international community to the response this year, though agencies say more is needed to keep the crisis from worsening.

Throughout the conflict, the army has been accused of human rights violations including unlawful detention, sexual abuse and extrajudicial killings.

The military also plays a key role in the northeast, particularly outside Maiduguri in the state of Borno which has been the worst-hit by Boko Haram.

Aid agencies mostly rely on the army and its convoys for access to other parts of the state and in many camps for displaced people it is the military distributing food and medical supplies.

In February, an air force strike on a refugee camp killed up to 170 people, among them at least six Red Cross aid workers. The military said the attack was an accident.

Reporting by Ahmed Kingimi in Maiduguri, Ardo Hazzad in Bauchi and Paul Carsten in Abuja; Additional reporting by Camillus Eboh, Alexis Akwagyiram and Felix Onuah in Abuja; Editing by Matthew Mpoke Bigg