Turkey attacked Kurdish insurgent camps in Iraq for a second night on Sunday, security sources said, in a campaign that could end its peace process with the Kurdistan Workers Party (PKK).
Ankara, which called for a special Nato meeting on Tuesday to discuss its security concerns, said two soldiers were killed and four wounded in an earlier attack by PKK militants.
Long a reluctant member of the US-led coalition against Islamic State, Turkey made a dramatic turnaround this week by granting the alliance access to its air bases and launching air raids against both the jihadist movement and the PKK.
It has no plans to send ground troops into Syria and the air strikes there are meant to give support to moderate Syrian rebels fighting Islamic State, Prime Minister Ahmet Davutoglu was quoted as saying by the Hurriyat newspaper.
He said the Syrian Kurdish PYD, which has links to the PKK, could "have a place in the new Syria" if it did not disturb Turkey, cut all relations with President Bashar al-Assad's administration and cooperated with opposition forces.
But the relapse into serious conflict between Turkey and the PKK has raised doubts about the future of Nato member Turkey's peace efforts with its own Kurdish foes that started in 2012, after 28 years of bloodshed, but have recently stalled.
Four Turkish F-16 fighter jets deployed from the Diyarbakir air base in the country's mainly Kurdish southeast hit PKK targets in Hakurk in northern Iraq, the sources told Reuters.
The strike came after a car bomb and roadside explosives hit a passing military vehicle on a highway near Diyarbakir overnight on Sunday, the army said. Kurdish militants then opened fire on the vehicle with rifle fire, it said. Two soldiers were killed and four were wounded.
At least six people had been detained in connection with the attack, Dogan news agency reported.
The renewed conflict has sparked protests in parts of Istanbul as well as the southeast. A police officer was killed in clashes in the city's volatile Gazi district on Sunday, the third day of violence there following the death of a leftist activist during police raids on suspected militants.
The PKK, which Ankara and Washington deem a terrorist group, has also targeted police officers in the southeast and elsewhere, accusing the Islamist-rooted central government of covertly helping Islamic State to the detriment of Syrian Kurds.
The outlawed PKK has waged an insurgency against Ankara for Kurdish autonomy since 1984. Opposition politicians and critics accuse President Tayyip Erdogan of taking up the campaign against Islamic State as political cover to clamp down on Kurds.
Davutoglu, who has said the operations will continue as long as Turkey faces a threat, discussed security with UN Secretary-General Ban Ki-moon in a telephone call overnight.
Nato said ambassadors will meet on Tuesday to discuss security at Turkey's request.
US denies connection
A senior US diplomat condemned recent PKK attacks but denied any link between Turkey's new strikes on Kurdish militants and its newfound boldness in tackling Islamic State, which has seized large expanses of neighbouring Syria and Iraq.
"There is no connection between these air strikes against PKK and recent understandings to intensify US-Turkey cooperation against ISIL," Brett McGurk, the deputy special presidential envoy for the coalition to counter Islamic State, said on Twitter, using one of Islamic State's acronyms.
White House spokesman Ben Rhodes, on an official visit to Kenya with President Barack Obama, told a news conference in Nairobi: "The US of course recognises the PKK specifically as a terrorist organisation. And so, again, Turkey has a right to take action related to terrorist targets. And we certainly appreciate their interest in accelerating efforts against ISIL."
Turkey said on Saturday its decision to enter the battle against Islamic State, soon after an IS suicide bomber killed 32 people, mainly Kurds, in the Turkish town of Suruc, would help create "a safe zone" across the nearby border in northern Syria.
Turkish opposition leaders say they are concerned that Erdogan wants the new attacks on the PKK to whip up anti-Kurdish sentiment before a possible early election later this year.
The Islamist-based AK Party he founded has until late August to find a junior coalition partner or face an early election. The AKP lost its single-party majority last June for the first time in more than a decade, in part due to the success of the pro-Kurdish Peoples' Democratic Party (HDP), which entered parliament for the first time.
"One of the aims of the air, land and media operations carried out right now is to undermine the HDP in early elections," HDP head Selahattin Demirtas said on Twitter.