A crater caused by a Turkish air strike against Kurdistan Workers Party (PKK) camps in the village of Amadiya, outskirts of Dohuk province. Reuters

AFP/Istanbul

Two more Turkish police were killed Friday in an attack blamed on Kurdish militants as tensions in the country boiled over amid the military's cross-border bombing campaign against extremists in Syria and Iraq.

Ankara has launched a two-pronged anti-terror offensive against Islamic State (IS) jihadists in Syria and Kurdistan Workers Party (PKK) militants at their bases in northern Iraq after a wave of deadly attacks in the country.

But so far air strikes had targeted mainly the PKK, which itself is bitterly opposed to IS, raising tensions in Turkey with its Kurdish minority.

Deadly attacks on the security forces blamed on the PKK have become a daily event, raising fears of a return to the dark days at the peak of its separatist insurgency in the 1990s.

In the latest violence, two Turkish police were killed in a gun attack on a police headquarters in the southern region of Adana.

Suspected PKK gunmen fired on police headquarters in the city of Pozanti, sparking clashes which left two police as well as two militants dead, the Anatolia news agency said.

According to an AFP tally, at least 13 members of the security forces have been killed in attacks blamed on the PKK since the current crisis erupted last week.

One worker from Turkey's railways monopoly was killed and one other person injured in a gun attack Thursday evening by suspected PKK militants in the Sarikamis district of the Kars region in eastern Turkey, reports said. 

They had been repairing damage from a non-fatal bomb attack on the line blamed on the PKK the day earlier.

Some 30 Turkish F-16 planes had Thursday launched a new wave of strikes against PKK targets in northern Iraq and Prime Minister Ahmet Davutoglu indicated there were no plans to wind down the campaign.

"The operations will continue until weapons are laid down, armed groups leave Turkey and Daesh stops being a threat in Syria," Davutoglu said, using an Arabic acronym for IS.

The army meanwhile issued a statement to express its thanks after "many" Turkish citizens applied to join the armed forces to "fight terrorism".

 Both HDP leaders probed

The first shots were also fired in what may prove a long-running legal process against pro-Kurdish political forces, with prosecutors announcing probes against the both co-leaders of the pro-Kurdish Peoples' Democratic Party (HDP).

Its charismatic male co-leader Selahattin Demirtas is accused of inciting people to take up arms during October 2014 protests that left dozens dead, Anatolia said.

If the case comes to court, he could face up to 24 years in jail, it added.

Meanwhile, prosecutors Friday announced a probe against its female co-leader Figen Yuksekdag -- the party splits its top post between a man and a woman -- for "making propaganda for a terror group".

The investigation relates to comments Yuksekdag made on July 19 supporting Syrian Kurdish groups which Turkey links to the banned PKK. 

The PKK's more-than-three-decade-long insurgency for greater rights and powers for Turkey's Kurdish minority left tens of thousands dead. The current violence has ripped apart a ceasefire declared in 2013.

 Erdogan ready for new polls

 Demirtas had accused President Recep Tayyip Erdogan of orchestrating the current security crisis in the hope of calling early elections to secure an outright majority for the ruling party after its lacklustre performance in June 7 polls.

With Turkey still without a permanent government as coalition talks drag on, Erdogan said early elections should be called "immediately" if the negotiations fail.

"It remains to be seen if a coalition will be formed," Erdogan told reporters on his plane while travelling on an official visit from China to Indonesia, the Hurriyet newspaper and other media reported.

"If not, we should turn to the national will immediately so that people will decide again and we will emerge out of the current situation," he said.