By Augustine Madu
KANO, Nigeria (Reuters) - Gunmen stormed a police station and a bank in a town in Nigeria's northwest, beyond a region covered by a military crackdown on a Islamist insurgency, a sign the offensive could provoke violence by smaller militant cells across the north.
It was not clear who carried out the attack.
Several gunmen were killed during a clash with police in the remote town in Katsina state, army spokesman Ikedichi Iweha told Reuters, without giving specific figures or police casualties.
Nigerian forces are trying to regain territory controlled by increasingly well-armed Boko Haram Islamist insurgents in their northeastern stronghold states of Borno, Yobe and Adamawa, which were put under a state of emergency by President Goodluck Jonathan on Wednesday.
Security experts believe a crackdown in the northeast could push insurgent attacks into other regions, or awaken smaller cells that operate in other parts of the north.
"It's difficult to tell if this is a criminal attack or part of another Islamist cell," one security source said.
"There have been incidents in the past in Katsina but it certainly hasn't been an insurgent stronghold."
Another security source said a bank was raided and prisoners were freed from the police station.
Boko Haram, other Islamist groups like al-Qaeda linked Ansaru and associated criminal gangs have become the biggest threat to stability in Africa's second largest economy and top oil producer.
Thousands have been killed since Boko Haram launched an uprising almost four years ago in an effort to create an Islamic state in a country of around 170 million split roughly equally between Christians and Muslims.
Violence has mostly happened far from economic centers such as the commercial hub Lagos or political capital Abuja and hundreds of miles away from oilfields in the southeast.
Military jets, helicopter gunships and thousands of troops are involved in the current offensive, which may answer some critics who accuse Jonathan, a southern Christian, of underestimating the severity of the crisis in the Muslim north.
Rights groups are concerned the state of emergency will lead to more abuses they have document by Nigerian forces.
(Additional reporting and writing by Joe Brock; Editing by Tim Cocks and Philippa Fletcher)