Top contenders for defence job
MICHELE FLOURNOY, 53
She served as the undersecretary of defence for policy from February 2009 to February 2012. If she does get the nod, she would be the first woman to hold the post.
In the past, she has been seen by some in the White House as too uncritically supportive of the uniformed military's perspective, suggesting that her appointment might set up Mr Obama for the kind of clashes he had with some military leaders over Afghanistan in his first term.
She is chief executive at the Centre for a New American Security (CNAS), a non-partisan think-tank that the Obama administration is thought to have relied on in developing national security policy. She co-founded CNAS in 2007 and served as its president until 2009, when she took the undersecretary job.
ROBERT WORK, 61
He is Deputy Defence Secretary and has served as undersecretary of the navy. A retired Marine colonel, Work also served as CEO of CNAS before the Senate confirmed him in April.
He has a reputation for being blunt, and as the Pentagon's No. 2 leader, Mr Work has had a major role in examining the Defence Department's budget and a variety of crises in recent months. For example, he is chairman of the Nuclear Deterrent Enterprise Review Group, which is assessing how the Pentagon should manage its ageing nuclear weapons arsenal in the light of several recent scandals.
ASHTON CARTER, 60
He served as the Pentagon's No. 2 official from October 2011 until December last year, stepping down after being bypassed in favour of Mr Hagel as defence secretary.
Like Mr Work, he oversaw efforts to reduce the Pentagon's budget and has been a fixture in the national security world for years. He joined the Obama administration in 2009 as the Pentagon's top weapons buyer and moved into the No. 2 job two years later.
Dr Carter, a former Rhodes scholar and Harvard professor, is seen as an effective communicator who understands the ins and outs of the Defence Department's unwieldy bureaucracy. But he has not been an influential political player, which could hurt his chances of being nominated for the top Pentagon job.