A security official in northeast Nigeria says soldiers have shelled suspected camps of Islamic extremists fighting in the region, killing at least 21 people.

The official said Friday that the fighting happened Thursday in the Sambisa Forest Reserve, just south of Maiduguri, the capital of Borno state. The official said that soldiers would remain in the area to secure it.

The official also said that Nigeria's government shut off mobile phone service to parts of northeast Nigeria as soldiers moved in to enforce an emergency declaration by President Goodluck Jonathan over three states there. Phone service was restored Friday, but the official said the phones likely would be shut off again.

The official spoke on condition of anonymity as he was not authorized to speak publicly about the operation.