Washington: Two former Pentagon officials, Ashton Carter and Michele Flournoy, seemed the likeliest candidates to succeed Chuck Hagel as head of the US defence department and direct the military campaign against the Islamic State (IS).

Soon after the resignation of the former Republican senator was announced officially by President Barack Obama following leaks to the press, it seemed the choice would be between Carter and Flournoy, who if chosen will be the first woman in history to head the defence department.

Senator Jack Reed was also mentioned as a possible replacement, but a spokesperson immediately stated that the Rhode Island Democrat did not want to be considered as a candidate to direct the Pentagon or any other agency of the Obama government.

Carter and Flournoy were already in the running when Obama nominated Hagel in January 2013 to lead the Pentagon in substitution of Leon Panetta, who decided to retire and return to his native California.

Flournoy, 53, was under secretary of state for policy between 2009-2012, first with Robert Gates as defence secretary and later with Panetta.

She left the Pentagon to attend to personal matters and is currently executive director of the Centre for a New American Security, a Washington think-tank.

Carter, 60, was No.2 in the Pentagon between October 2011 and December 2013, when he resigned for personal reasons and because he apparently felt uncomfortable taking orders from Hagel.

The current defence secretary will remain in the post until a successor is confirmed by the Senate.