KANO, Nigeria - Multiple blasts hit the northern Nigerian town of Gombe on Friday, witnesses said, and suspected Islamist gunmen killed five worshippers in a mosque attack in Nigeria's biggest northern city of Kano.It was unclear what caused the blasts, but setting off series of bombs has become a hallmark of an increasingly sophisticated insurgency by Islamist sect Boko Haram.The sect, which wants to impose sharia law across a nation of 160 million split evenly between Muslims and Christians, is spreading across the north of Africa's most populous nation."It started around 7:30 p.m. (1830 GMT) and it kept on going off," Jimam Lar, a PHD researcher doing field work in Gombe who heard the blasts from his hotel room, told Reuters by phone."I've heard several bombs. It's a good number. I can't say exactly how many, but it went on for over an hour at intervals," he said, adding that there was sporadic shooting as well in Gombe, a remote town that has been largely spared the violence in the north.Further northwest, in Kano, gunmen suspected to be Boko Haram opened fire on worshippers in a mosque, killing five, local police spokesman Majiya Musa said."They came on the back of a motorcycle and shot sporadically at worshippers this evening ... the situation is now under control. An investigation has been launched," he said.The Islamist insurgency used to be confined mostly to its heartland in the remote northeastern city of Maiduguri, but in the past six months has radiated out across the largely Muslim north and struck the capital Abuja a few times.It is increasingly plaguing Kano, a major trading center and ancient city that was once at the heart of the great caravan routes connecting Africa's interior with the Mediterranean.Boko Haram has replaced militancy in the oil rich Niger Delta to become President Goodluck Jonathan's number one security headache.Jonathan declared a state of emergency in parts of the north on Dec. 31, after a bombing in a Catholic church on the edge of Abuja claimed by Boko Haram killed 37 people and wounded 57. Yet the sect has proved hard to stamp out and has even seemed to expand its reach and the sophistication of its attacks.Its fighters killed 186 people in Kano, Nigeria's second largest city, last month, in a series of coordinated bomb blasts and shooting sprees that marked its deadliest attack so far.A handful of high profile arrests have been made, but nothing has yet stopped the flow of guns, bombs and militants that are wreaking havoc across northern and central Nigeria.In an interview with Reuters last month, Jonathan challenged Boko Haram to show their faces and state clearly there demands as a basis for dialogue.In the few public appearances its members have given, purported spokesmen for Boko Haram have declined the offer of dialogue unless all its members are released from prison, a highly unlikely outcome.