Pentagon grounded Turkey’s F-35 pilots at a U.S. air base in Arizona, the department told Reuters on Monday, in a move to curb Ankara’s involvement in the F35 jet programme after Turkey’s plans to purchase S-400, Russian air defence system. The U.S. says Turkey cannot buy both. “The department is aware that the Turkish pilots … are not flying,” Air Force Lieutenant Colonel Mike Andrews, a Pentagon spokesman, told Reuters. “Without a change in Turkish policy, we will continue to work closely with our Turkish ally on winding down their participation in the F-35 program,” he added. According to Reuters, another U.S. official who asked not to be named said the department halted the training over safety concerns.