The attacks came a day after more than 50 people were killed in neighbouring Bauchi state. The violence was not connected to unrest in the oil-producing Niger Delta in the south.
The four northern states are among the 12 of Nigeria’s 36 states that started a stricter enforcement of sharia in 2000 -- a decision that has alienated sizeable Christian minorities and sparked bouts of sectarian violence that killed thousands.
A senior member of the rebel group Boko Haram, which opposes Western education and demands the adoption of sharia law in all of Nigeria, threatened further attacks.
"We do not believe in Western education. It corrupts our ideas and beliefs. That is why we are standing up to defend our religion,“ Abdulmuni Ibrahim Mohammed told Reuters after his arrest in Kano state.
"Even if I’m arrested, there are more out there to do the job.“
The government estimates 55 have been killed in the two days of fighting, but estimates from security sources and residents add up to at least 80.
President Umaru Yar’Adua ordered heightened security in the affected regions and directed police to take all necessary action to contain and repel the militants.
More than 200 ethnic groups generally live peacefully side by side in the West African country, although civil war left 1 million people dead between 1967 and 1970 and there have been bouts of religious unrest since then.
CLASHES SPREAD
Members of the Boko Haram set several churches, a police station and a prison ablaze in Maiduguri, the state capital of Borno, residents said.
A Reuters reporter said he saw at least 20 bodies laid out on the street as his family was evacuating among hundreds of others. Gunshots and explosions could be heard throughout the city.
"The (militants) are right now moving into town. Riot police and soldiers are everywhere, but the boys are not afraid of them,“ said Gana Marari, another resident of Maiduguri.
In northern Yobe and Kano states, clashes between rebels and security forces killed at least four and injured 10. A Kano police spokesman said officers had arrested more than 100 gunmen.
Boko Haram, which means "education illegal“, began its string of attacks in the northeastern city of Bauchi on Sunday after the arrest of some of its members.
More than 50 Nigerians were killed and more than 100 arrested in those clashes, prompting the Bauchi state governor to impose a night-time curfew on the state capital city. There were no reports of violence in Bauchi on Monday.
