US Vice President Mike Pence has threatened Turkey’s future role in the NATO alliance, warning it against “reckless decisions,” like pressing ahead with the purchase of the Russian-made S-400 air defense system.

“Turkey must choose. Does it want to remain a critical partner in the most successful military alliance in history or does it want to risk the security of that partnership by making such reckless decisions that undermine our @NATO alliance?,” Pence tweeted on Wednesday, after making similar remarks at a NATO summit in Washington.

Turkey has insisted on purchasing the Russian weapons system, despite repeated warnings from the US. Even after the Pentagon halted deliveries of the fifth-generation F-35 fighter jet and related equipment to Turkey, Ankara remained defiant, with Foreign Minister Mevlut Cavusoglu telling the NATO summit that the purchase was “a done deal.”

Cavusoglu and US Secretary of State Mike Pompeo are currently meeting head-to-head at the summit to discuss the F-35 dispute.

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The Turkish FM said that operating the two weapons systems at once “will not be a threat” to the F-35 or other NATO systems. US lawmakers and military officials have consistently warned that Turkey’s deployment of the S-400 would give the Russian system opportunity to learn how to track and spot the F-35, with potentially deadly consequences for the jet in future conflicts.

Turkey is set to receive its first S-400 batteries this July. The US has thus far failed to lure its NATO ally away from the Russian system and to Raytheon’s more expensive Patriot missile system, despite offering it to Ankara at a knock-down rate earlier this year.

Turkey has also been a vital partner in the F-35 program, which is US’ most expensive military project to date. If both sides cannot reach a deal, the US will need to find new suppliers for parts of the fighter’s fuselage, landing gear, and cockpit displays.

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