Turkey has criticized a letter from Washington warning about Ankara’s removal from the F-35 fighter jet program, describing the language used in the message as unbecoming of a NATO ally.

Acting Defense Secretary Patrick Shanahan wrote a letter to his Turkish counterpart, Hulusi Akar, outlining how Ankara would be excluded from the F-35 program unless the Turkish government drops its plans to purchase Russian-made S-400 air defense systems.

Shanahan’s warning does not suit “the spirit of alliance,” Akar said according to a statement released by Turkey’s Defense Ministry and cited by Reuters.

He added that Turkey would send a formal response to the letter in the coming days.

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Akar noted that he will discuss the issue with Shanahan at a NATO meeting at the end of the month, but that Turkey’s position on the matter remains unchanged.

“We will continue to maintain the same attitude and stance,” he said.

Washington, meanwhile, has started to tighten the screws. The Pentagon confirmed on Monday that Turkish pilots have been banned from training on F-35s at Luke Air Force Base in Arizona.

The US has been pressuring Turkey for months to abandon its deal with Russia to purchase S-400 systems. Ankara has countered the demand by suggesting the creation of a joint working group to alleviate any possible US security concerns.

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Ankara has invested $1.25 billion into the trillion-dollar F-35 program, producing parts of the fuselage, landing gear, and cockpit displays for the jets. Turkey has ordered 100 units of the high-tech aircraft and says that it still expects the planes to be delivered.

According to Shanahan, Turkey has until July 31 to change course if it wants to remain in the F-35 program.

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