A Turkish F-16 fighter jet taking off from Incirlik airbase in the southern city of Adana, Turkey. Thomson Reuters

Turkish jets launched their heaviest assault on Kurdish militants in northern Iraq overnight since airstrikes began last week, hours after President Recep Tayyip Erdogan said a peace process had become impossible.

The strikes hit Kurdistan Workers Party (PKK) targets including shelters, depots, and caves in six areas, a statement from Prime Minister Ahmet Davutoglu's office said. A senior official told Reuters it was the biggest assault since the campaign started.

Iraq condemned the airstrikes as a "dangerous escalation and an assault on Iraqi sovereignty," saying it was committed to ensuring militant attacks on Turkey were not carried out from within its territory.

Turkey launched near-simultaneous strikes against PKK camps in Iraq and Islamic State fighters in Syria on Friday in what Davutoglu has called a "synchronized fight against terror."

Engaging in conflicts on two fronts is a high-risk strategy for the NATO member, leaving it exposed to the threat of reprisals by jihadists and Kurdish militants. Germany warned on Wednesday about possible attacks on Istanbul's underground rail network and bus stops.

Turkey has also opened its air bases to the US-led coalition against the Islamic State, also known as ISIS or ISIL, joining the frontline in the battle against the jihadists after years of reluctance. NATO gave Turkey full political support on Tuesday.

But Turkey's assaults on the PKK have so far been much heavier than its strikes against ISIS, fueling suspicions that its real agenda is keeping Kurdish political and territorial ambitions in check, something the government denies.

Of the 1,302 people arrested in what officials have described as a "full-fledged battle against terrorist groups" in recent days, 847 are accused of links to the PKK and just 137 to ISIS, government spokesman Bulent Arinc said.

Turkey has also made clear that its operations against Islamic State in Syria will not include air cover for Syrian Kurdish fighters also battling the jihadists.

The chairman of Turkey's pro-Kurdish opposition HDP party, Selahattin Demirtas, whose lawmakers Erdogan wants to see prosecuted for alleged links to the PKK, called for an immediate halt to violence on both sides.

"We have to establish democratic pressure that will help silence the guns immediately," he told reporters. "We are ready to work with all politicians who want to achieve this."

'Authoritarian system'

Kurdish children with a picture of Abdullah Ocalan, jailed leader of the PKK, during a celebration after it was reported that Kurdish forces took control of Kobani, in Aleppo. Thomson Reuters Turkish officials have said the strikes against the PKK are a response to increased militant violence in recent weeks, including a series of targeted killings of police officers and soldiers blamed on the Kurdish militant group.

Another soldier was killed on Wednesday and three seriously wounded in the eastern province of Agri in what appeared to be a PKK attack, the Dogan news agency reported. At least nine members of the security forces have been killed over the past week by suspected Kurdish militants.

The PKK has said the airstrikes are an attempt to "crush" the Kurdish political movement and create an "authoritarian, hegemonic system" in Turkey.

It has stopped short of explicitly pulling out of a peace process, though it said on July 11 that Turkey's construction of military outposts, dams, and roads for the armed forces' use had violated a cease-fire and that it planned to resume attacks.

Erdogan initiated negotiations in 2012 to try to end the PKK insurgency, largely fought in the predominantly Kurdish southeast, which has killed 40,000 people since 1984. The cease-fire, though fragile, had been holding since March 2013.

Western allies have said they recognize Turkey's right to self-defense but have urged it not to allow years of peace efforts with the PKK to collapse. While deeming the group a terrorist organization, Washington also depends heavily on allied Syrian Kurdish fighters battling ISIS.

On Tuesday, Erdogan said the process had become impossible and urged parliament to strip politicians with links to the militants of immunity from prosecution, a move seen as aimed squarely at the HDP.

Trust destroyed

Turkish President Tayyip Erdogan during a news conference in Sarajevo. Thomson Reuters Parliament began discussing the military operations in Iraq and Syria, as well as Erdogan's call for the lifting of immunity, in a heated session on Wednesday.

Deputy Prime Minister Yalcin Akdogan, who has overseen the government side of peace efforts, accused the HDP of sacrificing the process in the name of its own political ambition, saying it had been more interested in winning enough votes to enter parliament than in working to resolve the conflict.

The HDP won 13% of the vote in a June 7 election, helping to deprive the AK Party, which Erdogan founded, of its majority in parliament for the first time since 2002 and forcing it to seek a junior coalition partner or face a fresh election.

"The HDP has destroyed the trust, has betrayed the peace process," Akdogan told the state-run Anadolu news agency, stopping short of declaring it definitively over.

"It is unclear how the process will continue," he said. "First the PKK should put down weapons. If there's anything to be discussed, we can discuss it after that."

Many Kurds believe that by reviving conflict with the PKK, Erdogan is trying to undermine support for the HDP before a possible fresh election. He has made no secret of his desire to change the constitution and amass stronger powers, virtually impossible without a strong single-party AKP government.

The AKP has been holding coalition talks, but the leader of the largest opposition party, the CHP, said last week he saw an early election as the most likely outcome.

"The president does not want a coalition to be formed," said Idris Baluken, a senior HDP lawmaker. "He knows that if a coalition is formed, whatever remains from his executive presidency dream will completely be destroyed.

"He wants Turkey to have a snap election quickly."