The U.S. has reportedly stopped accepting Turkish applicants for a training program that teaches pilots to fly the new U.S.-manufactured F-35 fighter jets over a dispute with Turkey's government over purchasing a Russian air defense system.

Reuters reported Thursday that the Trump administration has put on hold plans to train further pilots beyond a handful currently being trained on the fighter jet systems at an Air Force base in Arizona.

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Two U.S. officials said that the decision could be reversed if Ankara backs down from plans to purchase a Russian air defense system which Pentagon officials say is designed to shoot down aircraft like the F-35, which they said would potentially allow Russia to gather data on the system's effectiveness against the F-35.

“The S-400 is a Russian system designed to shoot down an aircraft like the F-35,” said acting Assistant Defense Secretary Kathryn Wheelbarger, according to Reuters. “And it is inconceivable to imagine Russia not taking advantage of that [intelligence] collection opportunity.”

The Pentagon is reportedly in talks with Turkish President Recep Tayyip Erdoğan's government over the potential purchase of a Patriot missile battery as a substitute for the Russian defense system, which would satisfy the Defense Department's demands. Officials have said that the U.S. will not approve a potential $9 billion sale of F-35s to Turkey if the deal with Russia is approved.

Erdoğan, however, reportedly said this week that the Pentagon has yet to “[give] us an offer as good as the S-400s," referring to the Russian defense system.

"The United States has suspended deliveries and activities associated with the stand-up of Turkey's F-35 operational capability. Should Turkey procure the S-400, their continued participation in the F-35 program is at risk," the Pentagon said earlier this year.

"The United States has been clear that Turkey's acquisition of the S-400 is unacceptable," a spokesman added in early April. "We very much regret the current situation facing our F-35 partnership with Turkey, and the [Department of Defense] is taking prudent steps to protect the shared investments made in our critical technology."

The Senate Armed Services Committee last month said it is looking to block the sale of F-35 fighter jets to Turkey and cut Ankara from its partnership in the program if the NATO ally continues with its plan to buy a Russian missile defense system.