The U.S. and other NATO allies have expressed alarm for several years over the purchase of the S-400, with concerns that plugging the Russian system into the alliance could lead to data leaks and security breaches, including concerns it would allow Russians to gain information about the stealthy F-35 fighter.

Gen. Mark Milley, Trump’s choice to be the next chairman of the joint chiefs of staff, said in written testimony to the Senate Thursday that “my recommendation would be to discontinue the transfer of F-35 aircraft to Turkey and unwind Turkey from the F-35 program if Turkey accepts delivery of the S-400. The S-400 is a Russian system built to shoot down aircraft like the F-35.”