A powerful blast destroyed a police station in the northern Nigerian city of Kano, a senior police official said, the latest in a series of blasts in the country's second biggest city since Islamist insurgents stepped up their campaign there.
The explosives also affected some surrounding buildings. It was a big bang. For now, I can not say how many of our men are affected or whether the bomber died, the police source said.
Islamist sect Boko Haram claimed responsibility for a coordinated series of gun and bomb attacks in Kano on Friday that killed 186 people in their deadliest strike yet.
The new focus on Kano, an ancient city once at the heart of caravan routes connecting Africa's interior with the Mediterranean, underscores the sect's growing ambition. Gunfire was also heard there early on Tuesday, witnesses said.
From drive by shootings and petrol bombings in its northeastern heartland in Maiduguri, Boko Haram has spread across the north and have struck the capital Abuja.
The Islamists have killed at least 935 people since it launched an uprising in 2009, including more than 250 in the first weeks of this year, Human Rights Watch said on Tuesday.
Boko Haram, which means Western education is sinful in the Hausa language spoken in northern Nigeria, is loosely modelled on Afghanistan's Taliban. It has claimed responsibility for bombing churches, police stations, military facilities, banks and beer parlours in the mainly Muslim north of Nigeria.
The sect focuses its attacks mostly on the police, military and government, but has increased its attacks on Christian institutions. It says it is fighting enemies who have wronged its members through violence, arrests or economic neglect and corruption.
The United States-Nigeria binational security commission met on Tuesday. Discussions included the latest Boko Haram attacks and finding ways to stem the violence, diplomatic sources said. The commission usually meets at least once a year.
Boko Haram's attacks show a complete and utter disregard for human life, said Corinne Dufka, senior West Africa researcher at Human Rights Watch.
The Nigerian authorities need to call a halt to this campaign of terror and bring to justice those responsible for planning and carrying out these reprehensible crimes.
The report said 550 people were killed in 115 separate attacks by Boko Haram last year, mostly in the far northeastern state of Borno, where the sect was founded in 2002.
Boko Haram has moved from drive-by shootings and petrol bombs to suicide attacks using large and increasingly sophisticated explosives. A suicide car bomb last year killed 25 people at the United Nations headquarters in the capital Abuja.
In July 2009 the sect launched an uprising in the northeast in which more than 800 people were killed in five days of fighting with security forces.
The sect originally said it wanted sharia (Islamic law) to be applied more widely across Nigeria.
President Goodluck Jonathan has been severely criticised for not getting a grip on a group he says has infiltrated the police, military and all areas of government.
Jonathan's inability to respond effectively, or articulate a credible strategy, reinforces the growing perception of a deep leadership void in Abuja, London-based risk adviser Eurasia Group said in a research note on Tuesday.
So far militarisation of the region and strict curfews have only had limited effect and huge (military) spending outlays in 2012 offer little hope for a credible broader strategy.
(Reporting by Tim Cocks, Joe Brock and Mike Oboh; Writing by Joe Brock; editing by Tim Pearce)