Turkey will use the S-400 missile defence system it has bought from Russia despite the US threat of sanctions, a senior defence official said yesterday. 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. 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 the issue created “serious challenges” for Washington.