"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.
READ MORE: US-Turkey Fallout Over F-35, S-400 Buys Was Years in the Making
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.
READ MORE: Turkey Slams 'Unacceptable' US Bill Demanding Sanctions Over Purchase of Russia's S-400s
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.