BRUSSELS (Reuters) - Secretary of State Mike Pompeo on Friday 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.

U.S. Secretary of State Mike Pompeo attends a NATO foreign ministers meeting at the Alliance's headquarters in Brussels, Belgium April 27, 2018. REUTERS/Yves Herman

“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.

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.

Cavusoglu told Turkish broadcasters after the meeting that the S-400 deal was completed but that Turkey would be open to purchasing other defense systems from its allies.

“We have completed the S-400 process. That is a done deal,” he said. “But we need more air defense. We can discuss what we can do for further purchases.”

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

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 U.S. 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.

Cavusoglu said Turkey would not be deterred by possible sanctions.

“The ‘I will impose sanctions if you buy’ approach will not affect Turkey. Turkey will not accept this. If we are going to discuss what we can do together in the future, we are in.”