WASHINGTON, June 10 (Xinhua) -- Training of Turkish pilots for F-35 fighter jets in the United States was suspended, media reported Monday, as the United States began "unwinding" Turkey from the F-35 program due to the latter's purchase of Russia's S-400 air defense system.

Turkish pilots for F-35 fighters have been training at Luke Air Force Base in the U.S. state of Arizona.

"The department is aware that the Turkish pilots at Luke AFB are not flying," said a Pentagon spokesman, according to a Reuters report on Monday.

The move came days after the Pentagon announced last Friday that Washington would take measures to reduce Ankara's participation in the F-35 program.

"Turkey will not receive the F-35 if Turkey takes delivery of the S-400 system," Under Secretary of Defense Ellen Lord told reporters at a Pentagon briefing last Friday.

Lord noted that Turkish personnel currently in the United States for the F-35 training would be required to leave the country if Washington and Ankara fail to reach "a mutually-agreeable resolution" by the end of July.

The United States and its NATO ally Turkey have been sparring for months over the latter's purchase of the S-400, which Washington argued could gain valuable intelligence on the technical systems of the U.S.-made stealth F-35 jets when the S-400 is deployed on the Turkish territory.

Turkey suggested setting up a joint working group to address U.S. concerns about Ankara's purchase of the S-400, while the United States has not responded positively.

So far, Turkey has shown little sign of reversing its position. Turkish President Recep Tayyip Erdogan on June 4 reaffirmed his resolve to buy the S-400 despite threatened sanctions from the United States.