Turkish F-35 pilots are no longer flying at the Luke Air Force Base in Arizona as U.S. winds down Turkey's participation in F-35 program, the Pentagon told Reuters on Monday

"The department is aware that the Turkish pilots ... are not flying," said Air Force Lieutenant Colonel Mike Andrews, a Pentagon spokesman.

"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."

The Pentagon on Friday told Turkey that it was cutting off Ankara's purchase of F-35 fighter jets if the Turkish government goes ahead with plans to buy Russian-made S-400 missile defence system, ratcheting up what has been a lengthy, heated dispute between the two NATO allies.

In a sharp two-page letter, Acting Defence Secretary Patrick Shanahan said the training of Turkish pilots will end July 31 and Turkey would not be allowed to take final possession of the four F-35 aircraft it bought. Shanahan also warned that Ankara's purchase of the Russian system could hamper America's future relationship with Turkey, which has been a critical U.S. partner and base for combat operations, including for the war in Syria.

The timetable would allow pilots currently training on the F-35 to complete their training and for other pilots to be reassigned to other posts, Shanahan said. Four Turkish pilots are currently training at Luke Air Force Base. Two additional Turkish pilots are at the U.S. base working as instructors.

In addition to the six Turkish officers, there are an additional 20 Turkish aircraft technicians at the base undergoing training as well, according to the U.S. military.