Nigeria's President Goodluck Jonathan has said he sacked his security chiefs because new tactics are needed to fight Islamist group Boko Haram. He dismissed his defence minister and security adviser after about 150 people were killed last week. "[Boko Haram] are changing their tactics every day, so you also have to change your staff and personnel," he said in an interview on national TV. He was speaking after armed men freed 40 prisoners in Damaturu, in the north. It is not clear who carried out that but the city was one of those worst hit by last week's violence. Boko Haram has also previously staged several jailbreaks. 'Instigate religious crisis' President Jonathan said he would be happy to open dialogue with Boko Haram but not while the group was "faceless". "We must have a face to tell us why you are doing what you are doing and then of course we dialogue." He also urged Christians not to retaliate against Muslims after Boko Haram attacks, saying this was exactly what the Islamist group wanted. "Attacking churches is to instigate religious crisis," Nigeria's president said. "They believe that when they attack a church, Christian youths will revolt against Muslim youths. They don't care about who dies in the process. "If it doesn't work, the same Boko Haram will start attacking mosques to instigate Muslim youths to attack Christians. So they change their tactics." Scores of people died in revenge killings in Kaduna state last week after churches were bombed. Boko Haram has said it carried out the attacks Yobe state police commissioner Patrick Egbuniwe said his men were overpowered by at least 40 attackers at the Damaturu prison. He said there were no suspected Boko Haram members in the jail. No group has said it carried out the attack. The jailbreak comes after a week of violence in northern Nigeria including attacks on churches by Boko Haram which left about 150 people dead. Boko Haram, which means "Western education is forbidden" in the Hausa language, seeks to overthrow the government and establish an Islamic state. The group adheres to a strain of Islam that outlaws any kind of activity linked to Western culture. One of the group's demands is the release of all its jailed members.