US Secretary of State Mike Pompeo told his Turkish counterpart that the United States was seriously concerned over Ankara’s decision to buy Russian S-400 surface-to-air missile batteries, which are not compatible with NATO’s defenses, Reuters reported.

“The secretary underscored the seriousness of US concerns ... if they (Turkey) go ahead,” a senior U.S. official said after a meeting between Pompeo and Turkish Foreign Minister Mevlut Cavusoglu on the sidelines of a NATO foreign ministers session.

“He asked Cavusoglu to closely consider NATO interoperable systems,” the official added.

Hours after being confirmed as U.S. President Donald Trump’s new secretary of state, Pompeo headed to Brussels to participate in the NATO meetings, which have focused on Russian aggression and ways to strengthen the alliance.

During the meeting Pompeo also raised concerns about the detention of Pastor Andrew Brunson, who has been in jail since December 2016, and other Americans detained by Turkey.