Turkey's foreign minister says his country won't bow to "ultimatums" after the United States warned Turkey it was excluding it from the F-35 fighter jet program overs its deal with Russia to purchase its S-400 missile defense system.

U.S. Acting Defense Secretary Patrick Shanahan said in a letter to his Turkish counterpart last week that the training of Turkish pilots will end July 31 and that Turkey would not be allowed to take final possession of the four F-35 aircraft it bought.

Washington says the S-400 pose a threat to the F-35 jets.

The issue has strained already tense ties between the NATO allies.

Turkish Foreign Minister Mevlut Cavusoglu reiterated on Thursday that the S-400 deal had been concluded.

He said: "Turkey won't reverse its decision with such letters."