Gunmen have attacked two more police stations in Nigeria's second city of Kano, killing at least two people amid a wave of escalating violence blamed on Islamists.
One of the attacks occurred at dawn on Monday, setting off a gun battle with police, residents said. On Sunday night, gunmen stormed another police station near a bus station, leaving two civilians dead.
It was not immediately clear who was behind the violence, but Kano, the main northern city in Nigeria, has been targeted in recent weeks by attacks claimed by Islamist group Boko Haram.
The violence has included coordinated bombings and shootings on 20 January that left at least 185 people dead – the worst attack yet attributed to the group in Africa's most populous country.
Monday's attack was the second in three days against the police station in the Mandawari neighbourhood, underlining the apparent inability of the authorities to halt the violence.
"It was crazy. These guys came on motorcycles and opened fire on the police station but they met tough resistance from the police, and it lasted around 20 minutes," Jamilu Muhammad, who lives across the Mandawari station, told AFP.
Kano state police commissioner Ibrahim Idris confirmed the raid but said the gunmen "could not get access to the station," and were "repelled by officers". He made no mention of casualties.
"From my house I heard gunshots at exactly 5:50am (0450 GMT) coming from around the police station," said a local journalist who lives in the neighbourhood.
He said motorcycles were racing up and down his street as the sound of gunfire rang around the police station.
The attack occurred during Muslim morning prayers near the palace of the emir of Kano, the most important traditional leader in the city.
Dozens of soldiers went on patrol in the neighbourhood immediately after the assault but most later left, leaving only a truckload with seven soldiers outside the police station.
In Sunday's attack, a large group of gunmen converged on a police station in the Naibawa district, sparking a shootout with police that lasted more than 30 minutes, residents said.
"Two civilians were shot dead in the attack on the Naibawa police station," Idris told AFP on Monday.
He said the gunmen threw an explosive inside the police building, causing damage, but said no officers were hurt.
Kano remains under a dusk to dawn curfew following the series of bombings and shootings on January 20 that mainly targeted police stations and killed at least 185 people.
Monday's attack was the fourth on police buildings since then.
On Saturday, a purported Boko Haram spokesman ruled out dialogue with Nigerian authorities and instead warned it would spread its attacks to another northern city in Africa's most populous country.
In leaflets distributed around Kano over the weekend, Boko Haram warned residents that it would continue to target the security services in northern Nigeria's main city.
Kano had previously escaped the worst of Boko Haram's violence, and the brazen 20 January attacks highlighted the group's renewed strength.
The sect has been blamed for the deaths of more than 900 people in roughly 160 separate attacks since July 2009. It has claimed attacks that have killed more than 200 people since the start of 2012.
In a media interview last week, President Goodluck Jonathan said the group must make clear demands as a basis for dialogue, and calls have grown for talks to end the violence.
But Jonathan's call was "not sincere," self-declared Boko Haram spokesman Abul Qaqa told journalists by telephone in the northeastern city of Maiduguri, the group's base.
He warned that if members of the group being held by police in the northwestern city of Sokoto were not released, the group would launch attacks similar to those staged in Kano.
Boko Haram's intensified campaign of violence has shaken Africa's top oil producer and most populous nation, which is divided between a mainly Muslim north and mainly Christian south.