A group of six militants from the separatist Kurdistan Workers' Party (PKK) are believed to be behind Wednesday's ambush on a mountain road near the city of Kastamonu, the Taraf newspaper quoted unnamed police officials as saying.

The assailants hurled a hand grenade and opened cross-fire on a police car escorting a bus of the Justice and Development Party (AKP), killing an officer and wounding another, before fleeing into the forest.

The bus was taking AKP officials back to Ankara, while Prime Minister Recep Tayyip Erdogan had left Kastamonu by helicopter for another rally in nearby Amasya.

Erdogan also appeared to point an accusing finger at the PKK when he condemned the attack as the work of "associates of the separatist terrorist organisation" in remarks carried by Anatolia news agency late Wednesday.

Turkey holds parliamentary elections on June 12, in which the Islamist-rooted AKP will seek a third straight term in office.

The attack might have been a retaliation to the killing of seven PKK militants in clashes with the army in eastern Turkey last week, a senior police official told the mass-selling Hurriyet daily.

"We were expecting such attacks... We had issued warnings that the PKK would step up attacks in the election period," the unnamed official said.

The PKK, listed as a terrorist group by Turkey and much of the international community, took up arms for self-rule in the Kurdish-majority southeast in 1984, sparking a conflict that has claimed some 45,000 lives.

Northern Turkey is not a region where the PKK is usually active, but officials have previously said that several attacks in the area in recent years show the group is seeking a foothold there.

In February, the rebels threatened to end a unilateral truce, declared in August last year, while saying they would defend themselves "more effectively" against military operations.

A radical splinter group, which the PKK says acts outside its control, has contested the truce, and in November claimed responsibility for a suicide attack on a police patrol in central Istanbul that left 32 people wounded.