U.S. Defense Secretary Ash Carter says he believes his designated successor, if confirmed by the Senate as expected, will quickly have command of the office's responsibilities.

Carter declined to offer his view of whether President-elect Donald Trump's choice of retired Marine Gen. James Mattis undermines the principle of civilian control of the military.

Because Mattis has been out of uniform for fewer than seven years, which is the minimum required by law, his nomination will require new legislation to override the prohibition.

In an interview with reporters flying with him to Tokyo, Carter said questions about preserving the principle of civilian control of the military will be considered by the Congress.