US acting Defence Secretary Patrick Shanahan sent a letter to his Turkish counterpart last week, warning that Ankara would be removed from F-35 fighter jet programme if it accepts delivery of the S-400s.

Cavusoglu said such letters would not make Turkey back down from its decision.

The lira weakened to 5.8505 by 1008 GMT, down some 0.7pc from Wednesday's close. It stood at around 5.8320 before Cavusoglu's speech.