In the latest complication on an increasingly messy battlefield, Turkish warplanes launched multiple airstrikes on Tuesday against U.S.-backed Kurdish paramilitary forces that have been key to the fight against Islamic State in northern Iraq and Syria.

The Pentagon had yet to issue a formal statement on the strikes at deadline, but Turkish officials reportedly did not notify American commanders of the operation prior to the strikes in northern Syria and Iraq’s Sinjar region.

Syrian activists said the attack in Syria killed at least 18 members of the Syrian Kurdish militia known as the People’s Protection Units, while the Iraq airstrikes killed five members of the Iraqi Kurdish militia known as the peshmerga, another partner in the anti-IS coalition, the Associated Press reported. No U.S. forces in either country were harmed.

NATO allies Washington and Ankara have struggled to find a common strategy in the fight across the border in Syria, which includes a civil war against Syrian President Bashar Assad, whom Turkey has long opposed, and a battle against Islamic State, al Qaeda and other jihadist groups operating in the country.

“These airstrikes were not approved by the coalition and led to the unfortunate loss of life of our partner forces in the fight against ISIS that includes members of the Kurdish peshmerga,” said State Department spokesman Mark Toner.

Ankara characterized the strikes, which killed upwards of 70 Kurdish fighters, as a counterterrorism operation targeting members of the Kurdish Democratic Union party, or PYD. The group is the armed faction of the Kurdistan Workers’ Party, also known as the PKK.

Turkey categorizes both as terrorist groups, but Washington considers them as an integral part of the international coalition to defeat Islamic State.

Col. John Dorian, the top U.S. spokesman for coalition forces, confirmed that American commanders were largely left in the dark about the Turkish operation.

The Turkish General Staff said the strikes specifically targeted PKK positions near Sinjar mountain range in northern Iraq and Karacok range, north of the Syrian city of Al-Hasakah, according to an official statement.

The strikes, designed to stem the flow of weapons, men and materiel to PKK units in both countries, were executed “within the scope of the international law [and] with the aim of destroying the hotbeds of terrorism which target the unity, integrity, and safety of our country and nation,” the statement said.

Iraqi Prime Minister Haider al-Abadi slammed Ankara for its actions, saying the strikes were a violation of Iraqi sovereignty.

“Iraq is insistent on respecting the sovereignty of other countries. We call upon Turkey and all the other neighboring countries to respect the sovereignty of the Iraqi territory,” Iraqi government spokesman Saad Adisi told the Kurdish news outlet Rudaw.

Mr. Toner told reporters Tuesday that Ankara’s need to crack down on the PKK and its affiliates, which are responsible for a number of terror attacks inside Turkey, is not lost on U.S. officials.

“We’re cognizant of that and we’re also cognizant of the threat that the PKK poses to Turkey. But again, the point … is that Turkey cannot pursue that fight at the expense of our common fight against the terrorists that threaten us all. And that obviously means ISIS,” Mr. Toner said.

PKK fighters, alongside members of the Kurdish peshmerga, were integral in stemming ISIS’s bloody campaign across Syria and northern Iraq in 2014.

Kurdish paramilitaries in Syria have also been critical allies to American and coalition military advisers in the formerly ISIS-held territories of Manbij, al-Bab and elsewhere along the Turkish-Syrian border. Elements of the PKK-linked Kurdish militias are also integrated into the Syrian Democratic Forces, the U.S.-supported constellation of paramilitary groups, spearheading the attack on ISIS’s self-styled capital of Raqqa.

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