Pompeo addresses a press conference during a NATO Foreign ministers’ meeting in Brussels on April 27, 2018.

:: US Secretary of State Mike Pompeo on Friday warned of the “seriousness of U.S. concerns” in a meeting with his Turkish counterpart in Brussels over Ankara’s decision to buy Russian S-400 surface-to-air missile batteries, which are not compatible with NATO’s defenses.

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

“He asked Cavusoglu to closely consider NATO interoperable systems,” the official added. Hours after being confirmed as US 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.

$2.5 billion deal

Turkey signed an agreement with Russia for S-400 missiles, reportedly worth $2.5 billion, in late December as part of Ankara’s plans to boost its defense capabilities amid threats from Kurdish and Islamist militants at home and conflicts across its borders in Syria and Iraq.

Turkey’s purchase of the systems has unnerved NATO member countries, which are already wary of Moscow’s military presence in the Middle East, as the system is incompatible with the alliance’s systems.

NATO officials have warned Turkey about unspecified consequences of purchasing the S-400, but Turkish President Tayyip Erdogan has said ties with NATO remain strong.

On Thursday, three US senators introduced a measure to block the transfer of Lockheed Martin’s F-35 Joint Strike Fighter to Turkey, over what they said was Erdogan’s “path of reckless governance and disregard for the rule of law.”

Turkey plans to buy more than 100 of the F-35 aircraft. The bill would restrict the transfer of F-35s to Turkey and limit Ankara from receiving intellectual property or technical data needed to maintain and support the fighters.