The American general nominated to lead the NATO military alliance has told Congress that while Turkey remains an important ally, it should not get advanced U.S. F-35 fighter jets if it purchases Russian S-400 air defenses as planned.

“The S-400 and the F-35 are not compatible, and if Turkey proceeds down a path to procure and operate the S-400, they should not get the F-35,” Air Force Gen. Tod Wolters said at his Senate confirmation hearing Tuesday.