(CNN) The Pentagon warned Friday that there would be "grave consequences" for Turkey if it goes ahead with the purchase of the Russian-made S-400 missile system.

"If Turkey takes the S-400s there will be grave consequences," acting chief Pentagon spokesperson Charles Summers told reporters Friday, saying it would undermine America's military relationship with Ankara.

Summers said those consequences would include the US not allowing Turkey to acquire the F-35 jet and the Patriot missile defense system. This appears to be the first time the Pentagon has specifically said that Turkey will not be allowed to get the jets if the S-400 purchase goes ahead.

Turkey has been involved with the F-35 development for a long time, but its acquisition of the warplane has been opposed by some members of Congress amid the deteriorating relationship between Washington and Ankara over a range of issues.

Turkish pilots are currently training with the F-35 jets at Luke Air Force Base in Arizona.

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