The death toll from Friday's bomb attacks by Islamist militants in the Nigerian city of Kano is certain to rise further, doctors have said.Hospital officials say 160 people have been confirmed dead, but that bodies are still arriving at mortuaries.Boko Haram, which wants an Islamic state, said it launched the attack because the authorities refused to free a group of its members from jail.President Goodluck Jonathan visited Kano to offer his condolences.He has said that the security situation in Nigeria is now more complicated than it was during the 1967-1970 civil war.But he has promised to track down the perpetrators of the Kano attack.Boko Haram has launched a series of assaults over the past year, killing hundreds of people.Its members have bombed churches, government buildings and police stations - mostly in predominantly Muslim northern Nigeria.But the Kano attack appear to be the group's most deadly co-ordinated assault.One doctor told the AFP news agency that the final toll was likely to be about 250."Although the bulk of the bodies were brought here [the main hospital], others were deposited at three other hospitals," the doctor said.And aid workers were still collecting bodies from the streets on Sunday.The BBC's Andrew Harding in Kano says there is a growing belief that Boko Haram launched the attack to free some of its members from jail.A local police chief told our correspondent that about 50 Boko Haram attackers had managed to free "many" prisoners.Boko Haram, which loosely translates from the local Hausa language as "Western education is forbidden", says it wants to overthrow the national government and impose Islamic law.CurfewIt first hit the headlines in 2009 when a spate of attacks by its followers on police and government buildings in the city of Maiduguri led to a crackdown in which hundreds died.Since then, a wave of bombings and shootings have killed police officers, government officials and both Muslim and Christian civilians.Mr Jonathan's government has struggled to contain the violence, and has imposed emergency law in several parts of the north. Kano is under a night-time curfew.On Sunday in Bauchi state several people were killed in an attack on a police station.No group has said it carried out the attack, but Boko Haram have operated in the state in the past.