Turkish pilots might be excluded from participating in the F-35 training program, in yet another attempt by Washington to force its NATO ally out of the S-400 air defense system deal with Russia, according to a new report.

The US is “seriously considering suspending training for Turkish pilots on advanced F-35 fighter jets,” two anonymous sources told Reuters on Tuesday, without elaborating if the decision was already made.

Currently, four Turkish pilots and 47 military personnel are stationed in Luke Air Force Base in Arizona, where they are being taught how to handle the fifth-generation stealth jets. However, Ankara also has its servicemen training in Russia for the use of the S-400 air defense systems, first units of which are scheduled to be delivered to Turkey in the summer.

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The government of President Recep Erdogan maintains that it needs both F-35s and the Russian S-400 systems to protect its sovereignty in the troubled region. The US, however, is steaming ahead with its arm-twisting tactics trying to coerce Ankara to back down of $2.5 billion defense deal with Moscow.

Turkey has ordered 100 units of the F-35 and continues to insist that the stealth fighters must be delivered, especially since Ankara invested $1.25 billion into the trillion-dollar program, producing parts of the fuselage, landing gear, and cockpit displays for the jets.

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Insisting that a good NATO partner should only purchase US-approved weaponry, Washington announced a halt in delivery of equipment related to the F-35 to Turkey in late March. Last week, Ankara said that it was preparing for potential US sanctions which Washington has been threatening to impose under the Countering America’s Adversaries Through Sanctions Act (CAATSA).

Lockheed Martin ceremonially transferred the first two F-35 fighters to Turkey in June last year, and two more this April, but the planes still haven’t left US soil. Ankara had hoped to deploy the jets, which are still at Luke AFB, to Malatya air base in Turkey by November.

Also on rt.com Turkey will look for 'best technology' elsewhere if US blocks F-35 transfer

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