The United Nations says Nigerian soldiers have conducted an unauthorized search of a U.N. humanitarian base in the city where the Boko Haram insurgency began.

Edward Kallon, the U.N. humanitarian coordinator for Nigeria, expressed "grave concern" Friday over the military's actions in Maiduguri in the northeast.

Local media reported that large numbers of soldiers surrounded the U.N. building earlier Friday searching for arms.

The military confirmed the incident in a separate statement, saying the action was in line with the ongoing search efforts.

Rumors have spread that Boko Haram leader Abubakar Shekau may have been taking refuge in one of the U.N. buildings.

The U.N. says it has no information "regarding the reason or motivations for the unauthorized search."