The 68-year-old former senator and Vietnam war veteran was to join President Barack Obama at the White House later in the day to jointly announce his departure.

"In October, Secretary Hagel began speaking with the president about departing the administration ... Those conversations have been ongoing for several weeks," said an adminstration official, who spoke on condition of anonymity. 

"A successor will be named in short order, but Secretary Hagel will remain as defense secretary until his replacement is confirmed by the United States Senate."

The White House did not give any clue who might be Hagel's eventual replacement at the Pentagon, but the New York Times -- which broke the news of his departure -- cited three candidates.

Former under-secretary of defense Michele Flournoy -- who would be the first woman to run the Pentagon -- is in the running along with Senator Jack Reed of Rhode Island -- a former army officer -- and former deputy secretary of defense Ashton Carter, who served in the number-two role at the Pentagon, according to defense officials, who confirmed the report.

Hagel, as a Republican senator, voted in favor of the 2003 US invasion of Iraq, but later became a critic of the drawn-out conflict that ensued and was taken on by Obama early last year to oversee the withdrawal of US forces from Afghanistan.

Hagel's combat experience as a non-commissioned officer who was wounded in Vietnam was seen as a strength as he took on the job, but his public appearances have often appeared clumsy or underwhelming as the US administration struggles to adapt to new conflicts and articulate its strategies. 

"Over the past two years, Secretary Hagel helped manage an intense period of transition for the United States Armed Forces, including the drawdown in Afghanistan, the need to prepare our forces for future missions, and tough fiscal choices to keep our military strong and ready," the official said.  

"Over nearly two years, Secretary Hagel has been a steady hand, guiding our military through this transition, and helping us respond to challenges from ISIL to Ebola."

Although administration officials suggested the defense secretary had been forced to resign, a senior national security staff member in Congress told AFP that was not the case.

"Hagel quit," the staffer said. "Hagel found himself at odds with the administration."

Hagel's experience was similar to that of his predecessors, Robert Gates and Leon Panetta, who both complained after they left office of meddling by political appointees in the White House, the staffer said.

"It's related to the same complaints you heard from Gates and Panetta -- White House micromanaging every national security decision."

Senator John McCain, an outspoken critic of Obama's foreign policy, said he had spoken to Hagel by phone Monday. 

"I know that he was very, very frustrated," McCain said.

"Already the White House are leaking, 'Well he wasn't up to the job.' 

Believe me, he was up to the job."

McCain said the Obama administration had "no strategy" to fight the IS group and that Hagel had never been allowed into a White House inner circle making decisions.

Hagel had disagreed with the administration's approach to the Syrian regime, writing a two-page memo arguing for a more assertive stance towards President Bashar al-Assad, his aides recently disclosed.

Apart from the air war against the IS group, the White House also has come under criticism for the war effort in Afghanistan, with some Republicans questioning the timeline that will have all US troops out of the country by the end of Obama's term in two years.

Speculation about whether Hagel would remain as Pentagon chief gathered steam in October, with anonymous administration officials castigating his performance to a high-profile Washington Post columnist, David Ignatius.

Hagel was once considered a potential Republican presidential contender but in his current job, officials said he often said little at strategy sessions in the White House "situation room."

Earlier this month, Hagel called off a long-planned trip to Vietnam and Myanmar at the last moment, fueling questions about his future role.

In a television interview aired last week on the "Charlie Rose" show on PBS, Hagel repeatedly sidestepped questions about whether he would be kept on as defense secretary. 

"I don't get up in the morning and worry about my job," he said.  (AFP)