Three police students and one civilian have been killed and another 22 people wounded in an attack on a police bus in southeast Turkey.

Unknown assailants armed with guns and explosives attacked a white police bus on a busy street on the outskirts of the city of Diyarbakir on Wednesday.

At least 11 of the wounded were police students and the others were civilians, police said.

There was no immediate claim of responsibility for the daylight attack.

Television images showed bullet holes in the windows of the bus that was carrying the students.

One Turkish news agency said an unexploded grenade was found near the site.

Diyarbakir, the largest Kurdish city in the southeast, has a strong presence of Turkish soldiers.

The separatist Kurdistan Workers' Party (PKK) frequently target Turkey's armed forces and police in the area.

Recep Tayyip Erdogan, Turkey's prime minister, and the military have pledged to step up a campaign to destroy the PKK, including its fighters who are based across the border in Iraq.

Pressure has increased on the authorities after a cross-border attack by the PKK killed 17 soldiers on Friday, the worst attack against the Turkish military in a year.

Turkey blames the PKK, considered a terrorist organisation by the United States and the European Union, for the deaths of more than 35,000 people since it launched its armed campaign for an ethnic Kurdish homeland in southeast Turkey in 1984.