"The language in Shanahan’s letter contradicts the spirit of our alliance. We are preparing a fitting response", Akar was cited as saying by a Turkish news channel, NTV.
US Acting Secretary of Defense Patrick Shanahan sent a letter to Akar last week, seen by the Foreign Policy magazine, in which he said Turkish trainees would have to leave the United States by July 31.
Late last month, Acting Assistant Secretary of Defence for International Security Affairs Kathryn Wheelbarger said that the delivery of S-400s to Turkey would be "devastating, not only to the F-35 programme," but for "Turkish interoperability with NATO."
Russia and Turkey penned a $2.5 billion contract on the delivery of four battalion sets-worth of S-400s to Ankara in late 2017. Washington has issued a counteroffer, offering Turkey a $3.5 billion contract for a contingent of Patriot PAC-3s. Turkish officials have indicated that negotiations on the Patriot proposal are continuing, but have stressed that Ankara would not trade its S-400s for the US-made systems.
The first S-400s are expected to reach Turkish soil in July, with Turkey set to become just the fourth country after Russia, Belarus and China to possess the defensive platform. The S-400 is currently the most advanced mobile air defence platform in Russia's arsenal.