Russian S-400 system
© Sputnik / Vitaliy Ankov
Turkey's Foreign Minister Mevlut Cavusoglu rejected the US defense secretary's demand that Ankara cancel the purchase of Russian S-400 missile systems, despite threats it would be pulled from the F-35 fighter jet program.

"We reject the wording [of] the letter [by acting Defense Secretary Patrick Shanahan] to our defense minister. Nobody can give an ultimatum to Turkey," Cavusoglu stated on Thursday, echoing President Recep Tayyip Erdogan's earlier comments that the sale was already a done deal.

Taking note of Washington's concerns, Turkey has proposed measures to investigate whether or not the purchase of the Russian missile systems would actually pose a danger to NATO produced F-35s, including a joint working group with the US to study the issue. The US has so far snubbed the proposal.

Cavusoglu argued that even though President Trump himself was open to the idea, it had been shut down by other "institutions" within his administration.

Yesterday, Russia's ambassador to Turkey Aleksei Yerkhov reiterated his side's commitment to providing the military hardware, telling reporters "there is no change and will be no change" on the issue. Turkish staff is already undergoing training in operating the S-400 systems which are scheduled to begin being delivered this summer.