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.

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.

Ankara's push to buy the missile defence system from Moscow has caused significant tension between the NATO allies, whose ties are already strained by multiple issues including US support for a Syrian Kurdish militia viewed as terrorists by Turkey.

Despite US concerns over the S-400 system's interoperability with NATO equipment, Turkey has repeatedly said it would not back away from the deal.

The defence ministry statement stressed the importance of "continued dialogue based on mutual respect and friendship, and efforts to find an appropriate solution to the issues".

Turkish President Recep Tayyip Erdogan is expected to meet his US counterpart Donald Trump on the margins of a G20 summit in Japan later this month.

Erdogan at the weekend said Ankara expected the first deliveries of the Russian system in the first half of July.

Washington has also warned Turkey faces economic sanctions if it goes through with the deal.