The Turkish defence ministry said in a statement "the wording and approach" of a letter sent early this month by Acting US Defense Secretary Patrick Shanahan "was not in the spirit of an alliance (and) caused unease".
The ministry said Turkey's previously "known" opinions on acquiring the Russian S-400 missile defence system had been shared in a detailed manner, without offering further information, France24 reports.
Shanahan's letter warned Ankara has until July 31 to renounce the S-400 purchase or Turkish pilots would be booted from US training on America's latest-generation F-35 fighter jets.