At least 30 militants have been killed during air raids on their training camps in north-eastern Nigeria, officials say. An army spokesman said jets and helicopter gunships had been used to attack several camps. He said that a plane had been hit by anti-aircraft fire but had managed to return to base. States of emergency were declared this week in three north-eastern states hit by Boko Haram's insurgency. Nigerian forces are trying to regain control in the states of Adamawa, Yobe and Borno. Meanwhile, explosions and gunfire have been heard overnight in Katsina state. Residents have said that banks, police stations and prisons were destroyed in the town of Daura, near the border with Niger. They said they had seen the bodies of five members of the security forces and three militants, but there has been no official confirmation of casualties. Mobile phone networks were not functioning in many parts of north-east Nigeria on Thursday. Also a security official has said that the mobile phone service had been shut down during the military operation. Militants have previously attacked mobile phone masts in the area in an effort to disrupt communications.