The purchase of the Russian system and its subsequent delivery of the system in July has been a major source of friction between two NATO allies Turkey and the United States.

Last month the US said Turkey would be spared sanctions under a 2017 law if the S-400 system is not turned on.

"It is not a correct approach to say 'we will not use for someone else's sake' a system we had purchased out of our need and we paid that amount of money," Ismail Demir, the head of the Defence Industry Directorate, a government body, told private CNN Turk broadcaster.

"We will do our duty and (the system) will become usable. How it will be used is a decision to be made later," he said.

"We should respect the agreement we signed and that's what suits us as a country."

The issue was raised in talks in Washington this week between Turkish President Recep Tayyip Erdogan and his US counterpart Donald Trump.

Trump said afterwards that Turkey's controversial acquisition created "serious challenges" for Washington as he added officials would "immediately" get to work on resolving the issue.

Turkey was removed from the F-35 fighter jet programme as a consequence of the purchase.

Erdogan's spokesman Ibrahim Kalin on Friday said: "There is no question of a step backwards, Turkey will activate the S-400."