Turkey will never bow to US sanctions over its agreement to purchase Russian S-400 defence systems, Turkish Vice President Fuat Oktay said on Sunday in reference to a deal that has strained ties between the NATO allies.

Washington has said the S-400s could compromise the capabilities of its F-35 fighter jets - for which it has a separate deal with Turkey - and warned of possible US sanctions if Ankara pushed on with the Russian deal. Ankara has said the S-400s and F-35s would not impact each other and that it will not abandon the former.

Speaking in an interview with broadcaster Kanal 7, Oktay said US concerns on the issue were not reasonable and added that Turkey would not back down.