US Defense Secretary Chuck Hagel has resigned, ending nearly two years in the Pentagon's top job after a bruising mid-term election that undermined US President Barack Obama's Democrats in Congress.
Mr Hagel submitted his resignation letter after lengthy discussions with Mr Obama that began in October.
He had struggled to improve his ties with Congress after a contentious 2013 confirmation hearing
Mr Obama has accepted his resignation and said Mr Hagel is to remain in the role until his successor is confirmed by the US Senate.
The president said Mr Hagel told him last month it was time for him to complete his service.
Mr Obama said Mr Hagel has been candid with advice and counsel, and has "always given it to me straight".
In a statement this evening a US government spokesperson said Mr Hagel's disagreement with the White House on strategy in Syria had no impact on his departure.
Top potential candidates to replace Mr Hagel include Michele Flournoy, a former under-secretary of defense, and Ashton Carter, a former deputy secretary of defense, who were rumoured to be contenders for the job before Mr Hagel's was named to the role.
Senator Jack Reed, a Democrat from Rhode Island, is also considered a possible contender.
As a Republican senator, Mr Hagel voted in favour of the US invasion of Iraq, but later became a critic of the drawn-out conflict that ensued.
He was taken on by Mr Obama last year to oversee the withdrawal of US forces from Afghanistan.
Mr Hagel's experience as a non-commissioned officer who saw action in Vietnam was seen as a strength when he took on the job.
However, his public appearances have often appeared clumsy or underwhelming as the US administration struggles to adapt to new conflicts.
