"The S-400 deal is a done deal and we will not step back from this," he told a think-tank forum in Washington, where he is taking part in 70th anniversary celebrations of NATO.

The United States said April 1 that it was halting shipment of F-35 parts of Turkey as well as joint manufacturing work due to Ankara's purchase of the S-400 missile defense system from Russia.

The Turkish FM said his country turned to Russia as it could not buy U.S. Patriot missiles and quoted President Donald Trump as saying in an unspecified phone call that his predecessor Barack Obama had made a "mistake" not to sell the system to Ankara.

The Turkish foreign minister said the S-400 deal did not signal a warning toward Russia.

Reporter’s code: 50101