Turkey’s planned deployment of Russian S-400 missile defense systems would be “devastating” for the F-35 program to which Ankara has contributed over $1 billion, the Pentagon has warned.

“Completion of this transaction would be devastating, not only to the F-35 program, on which the West has placed its modernized integrated air capability, but it would potentially rupture Turkish interoperability with NATO, a key aspect of the defense of the alliance,” said Kathryn Wheelbarger, acting assistant secretary of defense for international security affairs.

Washington must not allow the $2.5 billion deal between Turkey and Russia to proceed, she told an audience at the Atlantic Council in Washington on Thursday, hinting that Russia cannot wait to get their hands on American secrets.

“Let’s be clear. The S-400 is a Russian system designed to shoot down an aircraft like the F-35, and it is inconceivable to imagine Russia not taking advantage of that collection opportunity,” she warned.

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Ankara has repeatedly stated that the S-400s will pose no risk to NATO systems, including F-35s, and has proposed creating a “joint working group” to dispel Washington’s panic. That proposal was once again discussed between Presidents Recep Tayyip Erdogan and Donald Trump during a phone call on Wednesday.

For now, the US wants nothing short of total obedience from its NATO ally. It has been applying massive pressure on Turkey, amid already strained relations, to back out of the Russian deal. Besides halting deliveries to Turkey of equipment related to the F-35s and preparing potential sanctions under the Countering America’s Adversaries Through Sanctions Act (CAATSA), Washington is insisting that Ankara should purchase its Patriot systems instead.

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Turkey, which has clearly stated that that S-400 purchase is a “done deal,” is demanding the delivery of its F-35 jets, four of which have already been ceremonially transferred but have yet to leave US soil. Turkey joined the program in 2002 and, over the years, invested more than $1.25 billion in developing and manufacturing various parts for the jet. Ankara believes that, without its participation, the program would just “collapse,” leaving Turkey with no other options but to seek fifth-generation fighters elsewhere, namely Russia.