The first meeting between the pair took place at a NATO summit in Brussels, where Isik warned Mattis about U.S. support for the PKK/PYD in northern Syria, the official said on condition of anonymity due to restrictions on speaking to the media.

“Anti-terror operations cannot succeed in this way,” Isik said, referring to the fight against Daesh. "A terrorist organization cannot be preferred to another one."

While Turkey considers the PKK/PYD to be a terrorist group affiliated to the PKK, which has waged war against Turkey since 1984, the U.S. sees the PKK/PYD as its ground ally against Daesh in Syria.

Isik also met counterparts from Spain, Italy, the U.K. and France, the official added.

Anadolu Agency