Chuck Hagel
Chuck Hagel is resigningSaul Loeb/AFP/Getty Images

Obama's decision to remove US defence secretary Chuck Hagel, as reported by the New York Times, is an acknowledgement that a new approach is now needed to face the growing threat from the Islamic State as well as growing tensions with Moscow.

The 68-year-old Republican's departure -- "under pressure" -- stems from the recognition that the Isis threat "requires a different kind of skills than those that Hagel brought to employ", said the daily.

Hagel first raised the idea of stepping down in talks with Obama in October and the midterm elections won by the Republicans were seen as a good transition point to make the change.

Before the threat of Isis became global, Hagel -- who had befriended Obama as a fellow critic of Bush's Iraq war -- was hired for the top post mainly to manage the delicate issue of the Afghanistan combat withdrawal and the shrinking Pentagon budget after the impact of the global financial crisis.

On the day that talks look set to be extended about sanctions for Iran over its nuclear programme, who are the possible candidates able to step in at such a sensitive time?

The three front-runners appear to be, the Vietnam combat veteran include the former undersecretary of defence Michèle Flournoy, senator Jack Reed, Democrat of Rhode Island and Ashton B Carter, former deputy secretary of defence.

Michèle Flournoy

Michèle Flournoy
Michèle FlournoyCNAS

A highly-influential figure, Flournoy Co-Founder and Chief Executive Officer of the Center for a New American Security (CNAS) thinktank that is said to have high ascendency in Obama administration's defence policies. In October, Flournoy was elected to the Intelligence Advisory Board (PIAB), which advises the president on intelligence matters.

Flournoy believes that the US military is "a force for good abroad" and campaigned for maintaining American forces in Iraq. She has criticised Obama administration's withdrawal plan from Afghanistan and is regarded as a liberal interventionist. She was undersecretary for defence from 2009 to 2012.

Jack Reed

U.S. Senator Jack Reed (D-RI) (L)
US Senator Jack Reed (D-RI) (L)Reuters

A former officer with the army's 82nd Airborne and West Point graduate, Reed recently defeated Republican Mark Zaccaria in Rhode Island almost unopposed. He's been senator for the state since 1997.

He previously served in the US House of Representatives from 1991 to 1997. Reed is regularly mentioned as possible secretary of defense, but always denied any interest for the job. In 2010, he reportedly turned down the post to succeed Robert Gates (the position was filled by Leon Panetta).

Ashton B Carter

: Deputy Secretary of Defense Ashton Carter
Deputy Secretary of Defense Ashton CarterGetty Images

A theoretical physicist and former Harvard professor, Carter has been deputy defence secretary from October 2011 to December 2013. During the Clinton administration, he was assistant secretary for international policy. The 59-year-old was responsible for the management of the defence department, around 2.2 million employees, but never hid his ambitions to lead the Pentagon, according to reports.

In his resignation letter, Carter just said that it was time for him to go.

Hagel's former number 2 joined the Obama administration in 2009 as the Pentagon's chief weapons buyer. Like Reed, he was considered a contender for the defence post when Panetta announced his retirement.