Kurdish militants killed two police officers in a car bomb attack on a checkpoint in southeast Turkey as authorities imposed a curfew in the region's largest city Diyarbakir where clashes broke out, security sources said.

Turkish forces backed up by helicopters and commandos shelled a mountainous area where the Kurdistan Workers Party (PKK) fighters had fled after the checkpoint attack in Sirnak province, killing six, the sources added.

A police officer was reported killed in another confrontation.

Hundreds of militants and more than 100 police and soldiers have died since a ceasefire collapsed in July, shattering a peace process launched in 2012. It is the worst violence Turkey has seen in two decades.

The Diyarbakir governor's office said it had placed the central historic Sur district under a round-the-clock curfew. Security sources said seven police officers were wounded in clashes there.

In other central areas of the city, police fired tear gas and water cannon at small groups of youths who threw stones and tried to set up street barricades in protest against the curfew.