The Turkish defence ministry said in a statement that "the wording and approach" of a letter sent 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 that it has until July 31 to renounce the S-400 purchase, or it would not be allowed to purchase around 100 cutting-edge F-35 fighter jets.

Turkey has been associated with the project almost since the beginning, with the state-owned Turkish Aerospace making part of the plane's fuselage.

Turkish pilots would also be booted from training programmes on the newest US combat plane, and Washington has warned that Turkey will face economic sanctions if it goes through with the deal.

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 will 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".

- F-35 alternative? -

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.

And in another show of defiance, Turkish officials unveiled a full-scale model of the country's own future stealth fighter, the TF-X, at the Paris Air Show this week.

"We have promised our country that it will be the best combat jet in Europe," the head of state-owned Turkish Aerospace, Temel Kotil, told journalists at the show's opening on Monday.

Ankara signed a cooperation accord to develop the jet with Britain's BAE Systems in 2017, and is aiming for a first test flight in 2025 with entry into service three years later.

Few analysts expect that ambitious deadline will be reached: Eight years elapsed between the first F-35 flight and its so-called "initial operating capacity", and the project has suffered multiple teething problems.

"The development of a strong and autonomous local defence industry is a priority for the Turkish government," Abdurrahman Can, the country's under-secretary for defence, told journalists in Paris.

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