Defense Secretary Jim Mattis expressed concern Monday over Turkey’s attack on Kurdish forces the U.S. has armed to fight the Islamic State.

Asked by reporters traveling with him to Asia whether he was worried about the situation in northern Syria, which pits NATO ally Turkey against U.S. ally the Kurdish YPG militia, Mattis replied, “We are very — yes.”

“Our top levels are engaged," Mattis told reporters on his plane en route to Hawaii. “Both Turkey and the American side. And we're working through it.”

The U.S. armed some YPG fighters who were part of the Syrian Democratic Forces that were assembled to defeat ISIS in Syria.

Mattis said Turkey notified the U.S. of its intent to move against the YPG, which Ankara considers a terrorist group, in an operation designed to establish a 20-mile buffer zone along Turkey’s southern border with Syria.

The advance word came in a telephone call from top Turkish military leaders to high levels of the American military.

“They warned us before they launched the aircraft they were going to do it, in consultation with us,” Mattis said. “And we are working now on the way ahead through the Ministry of Foreign Affairs, obviously on the mil-to-mil side, and we'll work this out.”

Mattis declined to say if the U.S. raised any objection to the Turkish offensive, and he was careful not to criticize Turkey publicly.

“It is the only NATO country with an active insurgency inside its borders. And Turkey has legitimate security concerns. We work very closely with Turkey,” Mattis said, who seemed to also suggest the YPG might constitute a genuine threat.

Mattis said with U.S. support the Kurdish fighters, including the YPG, “shredded” the Islamic State. The U.S. also ensured the Kurds used the U.S.-provided weapons only against ISIS.

“YPG had historic connections to the PKK. By having our officers and NCOs on the ground, we monitor that they are using what we are giving them for support to go against ISIS and they have proven their effectiveness,” Mattis said.

At a news conference in London, Secretary of State Rex Tillerson said while the U.S. is “concerned” about Turkey’s actions in northern Syria, “We recognize and fully appreciate Turkey's legitimate right to protect its own citizens from terrorist elements that may be launching attacks against Turkish citizens and Turkish soil from Syria.”

Tillerson said the U.S. is asking that both sides show restraint, and minimize the impact on civilians.

Turkey says its ground and air offensive, code-named “Operation Olive Branch,” is underway in the Afrin region, an enclave under Kurdish-control.

“If you look at Syria, it's easy to understand why Turkey has concerns about the chaos that Assad's handling of his people's protests has unleashed and then drawn all the terrorists in, into the chaos that he created,” Mattis said, referring to Syrian President Bashar Assad. “So we work with Turkey.”