U.S. Defense Secretary Chuck Hagel has resigned after nearly two years in the position.
President Barack Obama, speaking Monday at the White House with Hagel and Vice President Joe Biden at his side, said the defense secretary told him last month that it was time for him to complete his service.
Obama called Hagel a "steady hand" in helping his administration respond to the challenges of Ebola and Islamic State.
The president described Hagel as "no ordinary defense secretary" and said he has devoted himself to the United States' national security and its men and women in uniform for decades.
Hagel was the only Republican in Obama's Cabinet. He said serving as defense secretary was the "biggest privilege of my life." Hagel said he will remain in his position until his successor is confirmed.
He said that during his time as secretary, the defense department has "set the nation on a stronger course toward stability and prosperity."
The New York Times, citing senior administration officials, first reported Monday that the resignation comes "under pressure" amid several foreign policy crises, including the ongoing battle against the Islamic State militant group in Syria and Iraq.
But the White House says the decision to leave the Cabinet was mutual.
"The decision that was announced today is the result of conversations that the president and the secretary have been having for more than a month now," said White House spokesman Josh Earnest. "And as a result of these conversations the two of them arrived together at the determination that new leadership should take over at the Pentagon."
Hagel is the first Cabinet member to leave office since the Republicans won the Senate earlier this month, giving the party full control of Congress in January.
Hagel was sworn in as defense secretary in February 2013, succeeding Leon Panetta.
The Army veteran served in Vietnam before working in the public and private sectors. He was elected senator in 1996, representing the state of Nebraska as a Republican until 2009.
Former under secretary of defense Michele Flournoy is being mentioned in Washington as a frontrunner to replace Hagel.
Other possible replacements include former deputy defense secretary Ashton Carter and Robert Work, who currently holds that post.
VOA's Pentagon correspondent Carla Babb contributed to this report.