Turkish Defense Minister Hulusi Akar has promised that his country would cease its cooperation with the United States if no progress is made on the creation of a safe zone in Syria’s northeast.

In a recent phone call with his American counterpart Defense Secretary Mark Esper, Akar reiterated the longstanding request made by Turkey that the United States cut off its support for the predominantly Kurdish militia YPG, which the Turkish government accuses of being a Syrian branch of the Turkish PKK organization that has been designated a terrorist organization by multiple countries. The U.S. has been reluctant to accommodate Ankara since the YPG has been the Pentagon’s main partner on the ground in the fight against ISIS.

As a part of its attempt to balance its relationships with both Turkey and the YPG, the U.S. has ensured that the YPG withdrew to the eastern side of the Euphrates River. While this had previously tempered Turkish policy in Syria, it did not resolve the underlying opposition that is foundational to President Recep Tayyip Erdoğan’s long term policy objectives along the Syrian-Turkish border. Past months have seen repeated threats from Ankara about a renewed Turkish offensive inside Syrian territory.

As the second-largest military in NATO, the United States cannot risk losing support from Ankara. However, cracks have already emerged in the relationship. Earlier this year, the United States suspended Turkish participation in F-35 pilot training, as well as suspension of fighter jets sales already agreed upon, following Erdoğan’s refusal to cancel a purchase of Russian-made S400 missiles. The ultimatum issued was claimed to be on the basis that it exposes NATO to risk since it could potentially result in technical know-how being shared with the Kremlin. On the other hand, the offer of US-made Patriot Missiles suggests that it was partially economic in interest, with Trump’s two most recent defense secretary appointments having both come from the defense industry.