Attackers threw homemade bombs at a bus, a blast went off near a police station and two policemen were shot dead in a spate of attacks in the Nigerian city of Kano, officials said on Wednesday.

It was not clear who was behind the violence which began on Tuesday in Kano, the largest city in Nigeria's mainly Muslim north, though Islamist extremist group Boko Haram has carried out scores of such attacks.

It was not clear if the three incidents were connected. At least three people were also wounded.

Two men hurled homemade explosives at a bus said to be loaded with passengers, wounding two people and damaging the vehicle, a military spokesman said.

"Two IEDs were thrown at the bus which exploded and injured the driver and one other person in the vehicle," Lieutenant Iweha Ikedichi told AFP.

He said windows in the bus were shattered and one of its wheels burst, adding that no arrest has been made.

Later Tuesday near the scene of the bus attack, gunmen shot dead two policemen directing traffic at a roundabout.

"We lost two mobile policemen in an attack by some gunmen," a senior security source told AFP on condition of anonymity because he was not authorised to speak on the incident, adding that the assailants later fled.

A medical source at the Aminu Kano Teaching Hospital near the scene of the attack said two dead bodies in mobile police uniform had been brought to the morgue.

Residents also reported an explosion early Wednesday outside a police station in the city which injured at least one civilian.

They said the explosion across the road from a police station caused panic in the area as policemen on duty fired shots to fend off further attacks.

An AFP reporter said the area had been cordoned off by troops. An ambulance was also seen leaving the scene carrying one person with a bloodied face.

Kano was the scene of Boko Haram's deadliest attack yet in January, when at least 185 people were killed in coordinated bombings and shootings.

Violence linked to the Boko Haram insurgency is believed to have left some 3,000 people dead in Nigeria since 2009, including killings by the security forces.