President Barack Obama on Friday will announce his choice to replace Chuck Hagel as Pentagon chief, the White House said.
Favored for the post is Ashton Carter, a former deputy defense secretary under Leon Panetta from October 2011 to December 2013.
While stopping short of confirming the pick, the White House has hailed Carter's "detailed understanding" of the Pentagon's inner workings, and recalled that the Senate had unanimously confirmed his nomination as the Defense Department's number two in 2011.
However, said White House spokesman Josh Earnest: "It's actually the president's nominee, and he will announce it tomorrow."
Hagel, the outgoing Pentagon chief, announced his resignation last month. Officials privately said he was forced out after losing the confidence of the White House, as the United States wages an air war against Islamic State jihadists in Iraq and Syria.
At a news conference Thursday, Hagel rejected accounts that he was bundled out of his position and said it was a mutual agreement with the president.
"We both came to the conclusion that I think the country was best served with new leadership," Hagel said in his first news conference since his resignation announcement.
"But I think you have to know when to leave, too."
Carter, 60, has gained a reputation as an expert on hi-tech weapons and military budgets, portraying himself as a reformer intent on making the vast Pentagon bureaucracy more efficient.
But he has less experience overseeing war strategy and has never served in uniform, unlike Hagel, who was wounded in the Vietnam War.
When Carter stepped down last year, officials denied reports he had clashed with Hagel.
Former number three Pentagon official Michele Flournoy took herself out of the running for a post that would have made her the first woman to lead America's military. She cited family reasons.