"Turkey is a NATO ally. It's the only NATO country with an active insurgency inside its borders. And Turkey has legitimate security concerns," Mattis told reporters in a plane headed for Asia.
His remarks came amid Turkey's ongoing military operation against terror groups in the Afrin region.
Noting that the overall success against Daesh "does not remove many of Turkey's concerns," Mattis said it is "easy to understand" why Ankara is worried that the conflict will extend beyond the Syrian border.
”Turkey was candid,“ Mattis told reporters. "They warned us before they launched the aircraft that they were going to do it. We are very alert to it. Our top levels are engaged and we're working through it."
Turkey's operation was carried out against the PKK/KCK/PYD-YPG and Daesh terrorist groups under the framework of Turkey’s rights based on international law, the UN Security Council’s decisions and self-defense rights under the UN charter.
The PYD/PKK is the Syrian offshoot of the PKK terrorist group, which has been designated a terrorist organization by Turkey, the U.S. and the EU.
Since the mid-1980s, the PKK has waged a wide-ranging terror campaign against the Turkish state in which an estimated 40,000 people have been killed.