(Updates death tally, PKK spokesman)

DIYARBAKIR, Turkey, Oct 8 (Reuters) - Four police trainees and a civilian were killed and many other people were wounded in an attack on a police bus in southeast Turkey on Wednesday, the interior minister said.

Unidentified assailants armed with guns and explosives attacked the vehicle on a busy street on the outskirts of the city of Diyarbakir.

A spokesman of the illegal Kurdistan Workers' Party claimed responsibility for the attack.

"This incident happened in Diyarbakir inside the Turkish border this morning, where a Turkish patrol opened fire on PKK positions, so our fighters fired back and killed four of them and wounded more than 30," said PKK spokesman Ahmed Danees.

He said there were no casualties among PKK fighters.

Kurdish separatist guerrillas frequently target Turkey's armed forces and police in the mainly Kurdish southeast.

Interior Minister Besir Atalay told reporters that up to 19 people were injured in the attack and some of them were in critical condition.

The attack came shortly before parliament in Ankara approved a government request to extend a mandate to launch military operations against PKK rebels based in northern Iraq.

Television images showed bullet holes in the windows of the bus that was carrying the students. Attacks are rare in Diyarbakir, the largest city in the Kurdish southeast, where Turkish troops have a strong presence.

Anatolian state news agency said an unexploded grenade was found near the site.

Prime Minister Tayyip Erdogan and the military have pledged to step up a campaign to crush the separatist PKK, including those based across the border in Iraq.

The authorities have come under increased pressure after a cross-border attack killed 17 soldiers on Friday, the deadliest against the Turkish military in a year. The outcome of the vote had been widely expected.

Turkey blames the PKK, considered a terrorist organisation by the United States and the European Union, for the deaths of more than 40,000 people since it launched its armed campaign for an ethnic Kurdish homeland in southeast Turkey in 1984. (Additional reporting by Baghdad bureau; Writing by Ibon Villelabeitia and Selcuk Gokoluk; Editing by Charles Dick)

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