US Secretary of State Mike Pompeo on April 29 told Turkish counterpart Mevlut Cavusoglu that the US is seriously concerned by Ankara’s decision to buy Russian S-400 surface-to-air missile batteries. Nato member Turkey is going ahead with the acquisition even though the hardware is not compatible with the western military alliance’s defences.

“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 Cavusoglu on the sidelines of a Nato foreign ministers session in Brussels, Reuters reported. “He asked Cavusoglu to closely consider Nato interoperable systems,” the official added.

Turkey’s order for the S-400 batteries—unimaginable two years and a half ago when relations between Ankara and Moscow virtually froze after Turkish jets shot down a Russian fighter-bomber near the Turkish-Syrian border—followed a resetting of relations between Ankara and Moscow and a hard-nosed insistence from Turkish President Recep Tayyip Erdogan that Turkey will maintain relations with both the eastern and the western spheres as it sees fit.

On April 26, a bipartisan group of US senators moved to block Turkey from receiving F-35 Lightning II fighters over the imprisonment of American pastor Andrew Brunson. Donald Trump tweeted support for the on-trial Brunson on April 19.

Republican Senators James Lanford and Thom Tillis and Democrat Senator Jeanne Shaheen introduced a bill to prevent the transfer of the Lockheed Martin–made F-35 to Turkey and to block Ankara’s earmarked role to provide a maintenance depot for the aircraft.

Sensitive F-35 technology
Lanford reportedly described Erdogan, who is able to rule by decree under Turkey’s still-in-effect 22-month-long state of emergency, as continuing “down a path of reckless governance and disregard for the rule of law,” meaning the transfer of sensitive F-35 technology to his government would be “increasingly risky”. 
"Turkey’s strategic decisions regrettably fall more and more out of line with, and at times in contrast to, US interests,” he added.

Pompeo made mention of the Brunson case while in Brussels.

On April 23, it was reported that the future maintenance of the UK’s F-35 fighter jets might be put at risk by Washington’s displeasure with Ankara over its planned purchase of the Russian S-400 anti-aircraft systems.

The Register reported: “US government official Aaron Wess Mitchell threatened action if the Middle Eastern/ European nation completed its purchase of Russian S-400 anti-aircraft defence systems, according to Flight Global, which reported the assistant secretary of state for European and Eurasian affairs as saying during a US Congress session: ‘This is because Turkey is an F-35 customer. Testing F-35s against a Russian-made air defence system inevitably raises fears that vital information on neutralising the F-35 threat would then be handed to the Russians on a plate.’”

Turkey last December signed an agreement with Russia for S-400 missiles, reportedly worth $2.5bn.

Its foreign minister Cavusoglu told Turkish broadcasters after the meeting with Pompeo that the S-400 deal was completed but that Turkey would be open to purchasing other defence 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.”

He added: “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.”