US Secretary of State Mike Pompeo on Friday warned of the “seriousness of US concerns” over Ankara’s decision to buy Russian S-400 surface-to-air missile batteries, which are not compatible with NATO’s defenses. Pompeo met with Turkish Foreign Minister Mevlut Cavusoglu on the sidelines of a NATO foreign ministers meeting in Brussels. “The secretary underscored the seriousness of US concerns… if they go ahead,” Reuters quoted a senior US official as saying. “He asked Cavusoglu to closely consider NATO interoperable systems,” the official added. NATO officials had warned Turkey about unspecified consequences of purchasing the S-400, but Turkish President Recep Tayyip Erdogan has said ties with the alliance remain strong. Cavusoglu was reported Friday as saying that Turkey could evaluate offers for Patriot missiles or other air defense systems from NATO allies.