Turkish warplanes took off Sunday evening for a new wave of air raids on bases of the rebel Kurdistan Workers Party (PKK) in northern Iraq, according to Turkish media reports.

The first round of bombardments, which were unleashed late Friday, has threatened to unravel a fragile truce that has largely held since 2013.

Ambassadors of NATO states will meet on Tuesday at Ankara's request for talks on a spike in violence between Turkey, the Islamic State group and PKK Kurdish militants, the military alliance said Sunday.

"Turkey requested the meeting in view of the seriousness of the situation after the heinous terrorist attacks in recent days, and also to inform Allies of the measures it is taking," the NATO statement said.

The head of Turkey's main opposition party on Sunday said it was ready to go into coalition with the Islamic-rooted ruling party and end the political impasse that has persisted since June 7 elections.

The ruling Justice and Development Party (AKP) of President Recep Tayyip Erdogan failed to win an overall majority in the elections for the first time since it came to power in 2002, leaving Turkey facing either coalition government or early elections.

A Turkish policeman was killed on Sunday during clashes with leftist protesters in the the flashpoint Istanbul district of Gazi, the official Anatolia news agency reported.

Policeman Muhammet Fatih Sivri was shot in the chest from inside a building while he tried to make an arrest during clashes which have raged all day, it said. He was rushed to hospital but died from his wounds.

German Chancellor Angela Merkel on Sunday urged Turkish Prime Minister Ahmet Davutoglu to stick with the Kurdish peace process despite escalating violence, her office said.

Merkel assured Davutoglu of the "solidarity and support of Germany in the fight against terrorism" but also recalled "the principle of proportionality in the implementation of necessary measures."

The White House on Sunday backed Turkey's right to strike back at Kurdish militants, after waves of air and artillery strikes put a fragile ceasefire in jeopardy.

"Turkey has a right to take action related to terrorist targets," said deputy national security advisor Ben Rhodes after strikes against the outlawed Kurdistan Workers Party (PKK).

A car bomb attack killed two Turkish soldiers in the Kurdish-dominated southeast of the country, after separatist rebels warned they would no longer observe a truce after Ankara's air strikes on their positions in Iraq, officials said Sunday.

Turkey has launched a two-pronged "anti-terror" cross-border offensive against Islamic State (IS) jihadists and Kurdistan Workers Party (PKK) militants after a wave of violence in the country, pounding their positions with air strikes and artillery.

The Istanbul authorities on Saturday banned a planned anti-jihadist "peace march" scheduled to take place in the Turkish metropolis this weekend, citing security and traffic congestion.

The pro-Kurdish Peoples' Democratic Party (HDP) has hoped to rally thousands on Sunday for the protest to condemn violence by Islamic State (IS) jihadists following a suicide bombing on Monday that killed 32.

Turkey's military Saturday carried out a new wave of air and artillery strikes against both Islamic State (IS) jihadists in Syria and Kurdish militants in northern Iraq, putting in jeopardy a fragile ceasefire with Turkish Kurds.

The two-pronged operation against IS and the outlawed Kurdistan Workers Party (PKK) -- two groups who are themselves bitterly opposed -- came after a week of deadly violence in Turkey the authorities blamed on both organisations.

Turkish warplanes earlier Friday bombed three targets controlled by Islamic State (IS) jihadists up to 14 kilometers inside Syrian territory, the official Anatolia news agency reported.

Citing an official statement, it said that a three-story building used as a meeting place for IS jihadists in the El Tabiye area was hit 14 kilometers (8.7 miles) inside Syrian territory from the Turkish border.