The assailants opened fire on the bus, carrying employees of the local police academy, as it was passing through a residential area in Diyarbakir, the main city in the Kurdish-populated region, security sources said.

The victims were three police officers and the civilian driver of the bus, the governor's office said in a statement. All of the injured were police.

Initial suspicions fell on the Kurdistan Workers' Party (PKK), which has been fighting for self-rule in the east and southeast of Turkey since 1984.

Police immediately cordoned off the area where they also found an unexploded hand grenade.

An operation, supported by two helicopters, was underway to catch the attackers, the sources said.

The attack took place as parliament convened in capital Ankara to vote on extending the government's mandate to order strikes against PKK hideouts in northern Iraq.

Turkey charges that thousands of PKK rebels are holed up in the autonomous Kurdish-run north of Iraq where they obtain weapons and explosives for attacks on Turkish territory.

The PKK routinely targets Turkish security forces and although the fighting mainly takes place on rural ground in the southeast and east, the rebels have also been blamed for a series of attacks in urban centres.

In August, 16 people, most of them policemen and soldiers, were injured in a powerful car bomb blast in the western city of Izmir that was later claimed by a radical Kurdish group that Ankara says is linked to the PKK.

In the last bomb attack in Diyarbakir, in January last year, a powerful car bomb went off as a bus carrying soldiers was passing, killing seven people, six of whom were teenagers attending classes at a private school.