US defence secretary Chuck Hagel is resigning from Barack Obama’s cabinet, sources have revealed.
Mr Hagel, a former Republican senator, has served as Pentagon chief since early 2013.
Mr Obama is expected to announce the resignation later.
The move follows a tenure in which Mr Hagel has struggled to break through the White House's insular foreign policy team.
He is the first senior Obama adviser to leave the administration following the sweeping losses for the president’s party in the recent mid-term elections.
It also comes as the president’s national security team has been battered by multiple foreign policy crises, include the rise of the Islamic State militants in Iraq and Syria.
A senior defence official said Mr Hagel submitted his resignation letter to Obama on Monday morning and the president had accepted it.
Mr Hagel agreed to remain in office until his successor is confirmed by the Senate, the official said.
The official said both Mr Hagel and the president had “determined that it was time for new leadership in the Pentagon,” adding that they had been discussing the matter over a period of several weeks.
President Obama is not expected to nominate a new Pentagon chief today, according to one official.
Mr Hagel served as senator from Nebraska and became a critic of US involvement in Iraq. Mr Obama nominated him to succeed Leon Panetta as defence secretary in his second term.
Mr Hagel served in the Vietnam War and received two Purple Hearts.
Recent questions about Mr Hagel’s future at the Pentagon were prompted in part by his decision to postpone a long-planned trip this month to Vietnam.
At the time, officials said he needed to remain in Washington for congressional consultations, but that did not stop speculation that the White House might be looking for a replacement for the final two years of Mr Obama’s term.
Just last week Mr Hagel was asked about the speculation during an interview on the Charlie Rose show. He was asked whether he was concerned by the speculation.
“No. First of all, I serve at the pleasure of the president,” Mr Hagel said.
“I’m immensely grateful for the opportunity I’ve had the last two years to work every day for the country and for the men and women who serve this country.
“I don’t get up in the morning and worry about my job. It’s not unusual, by the way, to change teams at different times.”