Turkish air strikes pounded a group of Kurdish fighters allied to a U.S.-backed militia in northern Syria overnight, highlighting the conflicting agendas of NATO members Ankara and Washington in an increasingly complex battlefield.

The jets targeted positions of the Kurdish-led Syrian Democratic Forces (SDF) in three villages, northeast of the city of Aleppo, that the SDF had captured from ISIS, the Syrian Observatory for Human Rights said late on Wednesday.

The Turkish military confirmed its warplanes had carried out 26 strikes on areas recently taken by the Kurdish YPG militia, the strongest force in the SDF, and that it had killed between 160 and 200 combatants.

The British-based Observatory monitoring group reported a much lower toll of 11 dead and dozens wounded. Officials of the Kurdish-led administration that controls much of northeastern Syria said dozens had been killed.

A senior U.S. defense official said the groups struck by Turkish jets were not themselves U.S.-backed but were “close to and friendly with” the fighters Washington is working with.

Asked in the light of the air strikes whether he was concerned the U.S. alliance with Turkey was unraveling, Defense Secretary Ash Carter told a news conference at the Pentagon:

“With respect to Turkey, our partnership is very strong in the counter-ISIL campaign. We’re working with the Turks now very successfully to help them secure their border area.”

The United States has backed the Kurdish-led forces in their fight against ISIS, infuriating Ankara, which sees the YPG as an extension of Kurdish PKK militants who have waged a three-decade insurgency in southeastern Turkey.

Turkey views PKK fighters as terrorists.