Ambassador James Jeffrey, US Special Representative for Syria Engagement, recently announced that Turkey's purchasing of Russian S-400 surface-to-air defence systems remains the biggest snag in the relations between Washington and Ankara.
Speaking at an event held by Hudson Institute, Jeffrey explained that this issue is rather complicated because it involves the US fifth-generation F-35 stealth fighter project, as Turkey's actions allegedly undermine the aircraft's potential.
He noted, however, that US and Turkey see eye to eye on the majority of other issues, including NATO, the situation in Libya, Syria and Iraq, etc.
The United States claims that the S-400 missiles, delivered by Moscow to Turkey in July 2019, are incompatible with NATO defences and would jeopardise an agreement with Turkey for purchasing the US F-35 jets.
So far, Ankara has refused to make any concessions regarding the S-400 purchase.