Russian S-400 anti-aircraft missile launching system is displayed at the exposition field in Kubinka Patriot Park outside Moscow. (Photo: Alexander Nemeno/AFP)

Erdogan said this issue has nothing to do whatsoever with NATO or the F-35 project of the US. It was not about the S-400s but about Turkey taking actions on its own accord, especially in Syria, he added.

Washington has raised questions over the equipment's compatibility with the NATO's established weapon systems. The US believes the S-400 sale is part of Russian efforts to disrupt the alliance.