U.S. Air Force TSgt Arriel Bromley operates the fuel injection arm of a KC-10 Extender connected to an F-35Lightning II for a mid-air refueling training mission during exercise Astral Knight 19 over the Adriatic Sea, June 3, 2019. (Reuters photo)

ANKARA, June 8 (Xinhua) -- The Pentagon announced in a letter that it would halt F-35 fighter jet training for Turkish pilots over Ankara's purchase of the Russian S-400 air defense system, Turkey's state-run Anadolu Agency reported on Saturday.

In a letter to his Turkish Defense Minister Hulusi Akar, U.S. acting Defense Secretary Patrick Shanahan said that all Turkish pilots in the program must leave the country by July 31 and the training for new pilots will be suspended.

Turkish Defense Ministry also confirmed the letter in a statement on Saturday, saying that the Pentagon expected to address security concerns over the S-400 deal.

Shanahan expressed his expectation to find a solution to the existing problems between the two countries in the framework of the strategic partnership and to maintain comprehensive security cooperation, the ministry said.

The U.S. repeatedly warned that it will cut off Ankara's purchase of F-35 fighter jets if the Turkish government goes ahead with plans to buy Russian S-400 air defense system, triggering a heated dispute between the two NATO allies.

Washington has already suspended deliveries of parts and services related to Turkey's receipt of the multi-million-dollar jets.

However, Turkish President Recep Tayyip Erdogan on Tuesday reaffirmed his resolve to buy S-400 system despite threatened sanctions from the United States.