Turkish pilots have been banned from training on F-35s at Luke Air Force Base in Arizona, the Pentagon confirmed, as Washington steps up pressure on Ankara to force it out of S-400 air defense deal with Russia.

“The department is aware that the Turkish pilots ... are not flying,” Pentagon spokesman, Air Force Lieutenant Colonel Mike Andrews, told Reuters.

Without a change in Turkish policy, we will continue to work closely with our Turkish ally on winding down their participation in the F-35 program.

Four Turkish pilots and 47 personnel were undergoing training at the facility in Arizona, where they were instructed by the US Air Force on how to maintain the fifth-generation jets – four of which have already been ceremonially transferred to Turkey, without leaving the US soil though. Turkey planned to deploy the first batch of fighters by November, alongside the Russian S-400 air defense systems that are due for delivery in summer.

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While Ankara argues it has a sovereign right to diversify its defense suppliers, Washington demands full obedience and insists that the government of President Recep Tayyip Erdogan should only purchase NATO-approved weapons systems.

Turkey has repeatedly stated that the $2.5 billion S-400 purchase from Russia is a done deal, but at the same time stressed that it is willing to create a joint working group to alleviate any possible US security concerns. 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. Given the country's significant role in developing the fifth-generation fighter, Turkey continues to insist that all 100 units it had purchased must be delivered.

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Earlier, acting US Secretary of Defense Patrick Shanahan sent a letter to his Turkish counterpart Hulusi Akar, stating that Turkey’s participation in the F-35 program will be effectively suspended as of July 31, unless Turkey changes its attitude. Ankara is currently drafting its response to Washington’s concerns, head of the Turkish Defense Industries Directorate Ismail Demir said Monday.

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