Turkish President Tayyip Erdogan confirmed on Friday that Turkey had been testing the S-400 air defence systems that it bought from Russia and said U.S. objections on the issue did not matter. Washington says Ankara’s purchase of the Russian systems compromises NATO defences, and has threatened sanctions. An apparent firing test of S-400s test last week prompted a furious response from the U.S. State Department and the Pentagon. “(The tests) have been and are being conducted. The United States’ stance absolutely does not concern us. If we are not going to test these capabilities at our disposal, then what are we going to do?” Erdogan told reporters. The two NATO allies have long been at odds over the S-400s and Washington reacted last year by suspending Turkey from its F-35 jet programme. Turkish officials have said the systems will not be integrated into NATO’s defence infrastructure.