President Trump and Turkish President Recep Tayyip Erdogan the 2018 NATO Summit in Brussels, Belgium. Photo: Sean Gallup/Getty Images

The U.S. will block Turkey from purchasing F-35 aircraft and halt the training of Turkish military pilots, the AP reports. The move comes in response to Turkey's plan to buy an advanced Russian S-400 missile defense system.

Our thought bubble, per Axios' Dave Lawler: Turkey is a NATO ally, but relations with the U.S. have deteriorated sharply amid conflicts on a variety of issues, including Syria. Prime Minister Recep Tayyip Erdogan has hedged his bets by growing closer to Moscow, and antagonized Washington in the process. This relationship is on increasingly shaky ground.

Details: Turkish pilots will no longer be trained after July 31, according to a letter from acting Defense Secretary Patrick Shanahan. All Turkish personnel involved in the program will also have to leave the U.S. by the end of July.

The backdrop: The U.S. has exerted pressure in recent months over the purchase of the Russian missile system, which is incompatible with NATO systems, but Erdogan has remained defiant. Shanahan laid out some of the consequences of that decision in his letter:

“In addition to threatening the security of platforms like the F-35, Turkey’s procurement of the S-400 will hinder your nation’s ability to enhance or maintain cooperation with the United States and within NATO, lead to Turkish strategic and economic over-dependence on Russia, and undermine Turkey’s very capable defense industry and ambitious economic development goals."

Go deeper: Turkey defiant in face of U.S. pressure over Russian missile system

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