White House spokesman Josh Earnest, asked about criticism that the White House micromanages the Pentagon, said there was "always some natural tension" between the White House and the Defense Department.

"The president is, after all, the commander-in-chief. So he obviously has significant say over what kinds of things are happening over at the Department of Defense," Earnest said.

The White House had no comment on Carter's selection.

But spokesman Earnest had glowing praise for a man little known outside Washington, but who has a reputation inside the capital as a career technocrat and sharp intellect who is unlikely to shake up US military strategy.

Carter performed "very, very ably" in his prior Pentagon appointment, Earnest said at his daily briefing, noting that the Senate had confirmed him unanimously. Previous Senate confirmation is a valuable asset for a nominee seeking to clear partisan gridlock on Capitol Hill.