Nigeria's Maiduguri city rocked by explosion
An explosion and heavy gunfire has rocked the northern Nigerian city of Maiduguri - a stronghold of the Islamist Boko Haram group.
Its fighters threw an explosive device at a military patrol, wounding three officers, military officials said.
Soldiers then engaged in running gun battles with the militants in the centre of Maiduguri.
The BBC's Bilkisu Babangida in Maiduguri says the city is in lockdown, with residents staying indoors.
The Boko Haram sect says it is fighting for Islamic rule, and campaigns against all activity associated with the West.
At least 40 people have died in Maiduguri, the capital of Nigeria's Borno state, in the past 10 days in attacks blamed on the sect.
Meanwhile, suspected Boko Haram gunmen attacked a police station in the town of Toro in nearby Bauchi state on Tuesday night, escaping with an arms cache, correspondents say.
The latest trouble came as the former governor of Borno state, Ali Modu Sheriff, issued a public apology to Boko Haram for his role in the brutal crackdown on the sect in 2009.
The sect demanded the apology as part of conditions for a truce with the government.
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The head of the military task force in Maiduguri, Major General Jack Nwaogbo, told the BBC his officers had been targeted by an improvised explosive device while on patrol in the city.
Three of them had been wounded, although not seriously, he said.
Our reporter says the explosion took place at 0700 local time (0600 GMT) in the Abba Ganaram area, where the Boko Haram group had its headquarters until it was destroyed in a crackdown by the security forces.
The explosion shook her home, more than 5km (three miles) away, our reporter says.
Soldiers then pursued the attackers, exchanging heavy gunfire with them through the streets of Maiduguri.
Some of the fighting has taken place near the palace of the Shehu of Borno, the state's most revered Muslim leader.
Our reporter says eyewitnesses told her that cars parked near the area had been smashed in the clashes.
The militants have disappeared into the neighbourhood and soldiers are carrying out house-to-house raids to arrest them, she says.
Shops and markets have closed and people are afraid to venture out of their homes, our reporter adds.
Two years ago, Nigeria's security forces brutally suppressed an uprising by Boko Haram, destroying their compound and then killing their leader, Mohammed Yusuf, in custody.
Instead of disappearing, the group has re-emerged and vowed to avenge their leader's death.
Mr Sharif, who stood down as governor of Borno in April, took out full page adverts in two national papers on Tuesday and Wednesday to apologise for the crackdown.
"I wish to publicly tender my apology… I might have offended in the course of discharging my duties as the executive governor of Borno state," AFP news agency quotes Mr Sheriff as saying in the advert.
"It is human to err but divine to forgive," he added.
His apology follows that of the former governor of neighbouring Gombe state and the current governor of nearby Bauchi, who recently issued statements of regret about any role they may have played in human rights violations against the sect's members.
Recently Boko Haram, which has assassinated dozens of security officers, politicians and religious leaders in the last 10 months, demanded that all three issue apologies to the group.
Last month, the sect also attacked the police headquarters in the capital, Abuja.