KANO, Nigeria (AFP) - The death toll from the latest gun battles between militants and police in northern Nigeria, wracked by Islamist and ethnic violence, rose to 18 on Wednesday, police said.

Gunmen attacked five police posts and a prison across three cities late Tueasday, sparking responses from security forces, with the heaviest fighting concentrated in Kano, Nigeria's second city and the largest in the north.

No group claimed the raids, but the violence was likely to be blamed on Boko Haram Islamists, responsible for more than 1,000 deaths in Nigeria since mid-2009.

The militants launched gun and bomb attacks on Kano's Dala, Panshekara and Challawa police posts late Tuesday, state police commissioner Ibrahim Idris told AFP. The Goron Dutse prison was also targeted.

"All these attacks were repelled," he said. "A total of 17 extremists were killed by our men. We lost a police corporal."

He added that three suspects were arrested and were being interrogated, while officers recovered 14 improvised explosive devices as well as rifles, ammunition and vehicles following the clashes.

Kano was the site of Boko Haram's deadliest attack yet, when coordinated bombings and shootings left at least 185 people dead in January.

In Taraba state, residents said gunfire and explosions broke out late Tuesday and continued into Wednesday morning. A police spokesman confirmed there was an attack on a regional police headquarters.

Taraba is located near northeastern states badly shaken by Boko Haram violence. It also neighbours Plateau state, hit by waves of ethnic violence as well as Islamist attacks.

The Islamist group's insurgency, concentrated in Nigeria's mainly Muslim north, has frequently targeted the security forces, the though the group has attacked churches and other symbols of authority.

It has recently taken to attacking Christians worshipping on Sundays, leading to deadly reprisal violence from Christian mobs in the city of Kaduna earlier this month and sparking fears of wider unrest.

Nigeria, Africa's most populous nation and largest oil producer, is roughly divided between a mainly Muslim north and predominantly Christian south.

On Sunday, President Goodluck Jonathan said Boko Haram was seeking to incite a religious crisis by attacking churches in an attempt to destabilise the government.

Jonathan, who has come under heavy criticism in recent days over the spiralling violence, described how the group had moved from targeting local rivals to government institutions and now churches.

On Friday, he fired his national security adviser and defence minister.

There have been growing warnings that there could be more cases of residents taking the law into their own hands if something is not done to halt Boko Haram's attacks.

The group initially said it was fighting for the creation of an Islamic state, but its demands have since shifted repeatedly. It is believed to have a number of factions, including a main Islamist wing.

Many say deep poverty and frustration in the north have been main factors in creating the insurgency.

The United States on Thursday said it had designated the head of the main branch of Boko Haram, Abubakar Shekau, a "global terrorist" along with two others tied to both Boko Haram and Al-Qaeda's north African franchise AQIM.