The Pentagon has escalated its threat to cancel Turkey's purchase of F-35 fighter jets and stop training Turkish pilots if the country goes ahead with buying Russia's S-400 missile defense system.
On Friday, acting Secretary of Defense Pat Shanahan sent a letter to his Turkish counterpart outlining how Turkey would be removed from the program if it went ahead with the purchase. One of those steps includes ending the training of Turkish pilots by July 31.
U.S. officials are concerned that Russia's S-400 air defense missile system could pinpoint the weaknesses of its Lockheed Martin F-35 stealth fighter jets.
Shanahan's letter said there was still time for Turkey to take the steps necessary to stay in the F-35 program. This seems unlikely, since Turkey's president said Tuesday that it would be "out of the question" for the country to cancel its deal with Russia.