The attack came as the parliament extended the government's mandate to order cross-border strikes on Kurdistan Workers' Party (PKK) militants using northern Iraq as a springboard for attacks on Turkey as part of a 24-year separatist campaign.

The assailants opened fire on the bus -- carrying employees of the police academy -- from two different positions as it was passing through a residential area in Diyarbakir, the main city in the Kurdish-populated region, Interior Minister Besir Atalay told reporters in Ankara.

The victims were four police officers and the civilian driver of the bus, he added. All of the injured were police officers.

Hospital sources said six had sustained heavy injuries and were being operated on.

Speaking after the vote in parliament, Prime Minister Recep Tayyip Erdogan suggested the attack was the work of the PKK and said Ankara was determined to rout the rebels.

"These (attacks) will not daunt us. We will continue our struggle until the terrorist organisation lays down arms," Erdogan said, using the official jargon for the PKK, blacklisted as a terrorist organisation by Turkey, the United States and the European Union.

President Abdullah Gul also condemned the violence in a written statement and said: "Those who carried out this treacherous attack against our security forces ... will definitely be brought to justice."

After the attack, police immediately cordoned off the area, where they also found an unexploded hand grenade. News reports suggested that the assailants had thrown it under the bus.

An operation, supported by two helicopters, was underway to catch the attackers, security sources said, adding that police were questioning six people. It was not immediately clear whether they were suspects.

Wednesday's assault comes days after PKK rebels killed 17 soldiers in a deadly attack on a military outpost in the province of Hakkari near the Iraqi border, in the bloodiest attack this year.

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.

Some 44,000 people have been killed since 1984 when the PKK took up arms for self-rule in Turkey's east and southeast.

Ankara accuses Iraqi Kurds of tolerating the PKK presence in northern Iraq and has repeatedly urged them to take tougher action.

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.

Authorities suspect the PKK to be behind two bomb attacks in a crowded street in Istanbul in July, which killed 17 people, among them five children, and wounded more than 150.

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