Hagel resigns as US Defence Secretary, official says

US Defence Secretary Chuck Hagel resigned on Monday, leaving under pressure as President Barack Obama faces critical national security challenges, including fighting Islamic State in Iraq and Syria and revising plans to exit Afghanistan.

It was the first major change to Obama's Cabinet since his Democrats were routed in midterm elections three weeks ago, and Republicans, who took over the Senate and now control both houses of Congress, are looking to bring changes.

Hagel was appointed less than two years ago as Obama pushed his signature programme of winding down wars in Afghanistan and Iraq, a process now being upended with US re-engagement in Iraq and greater military cooperation with Kabul.

Hagel had privately expressed frustration to colleagues at the administration’s strategy toward Iraq and Syria and at his lack of influence over the decision-making process, a source familiar with the situation said.

US officials said publicly the decision was mutual but privately others said he had been forced out.

"There’s no question he was fired," said one with knowledge of the matter.

He raised questions about Obama's strategy toward Syria in a two-page internal policy memo that leaked this fall in which he warned that Obama's policy was in jeopardy due to its failure to clarify its intentions toward Syrian President Bashar al-Assad.

Obama has insisted the United States can go after Islamic State militants without addressing Assad, who the United States would like to leave power.

Republicans demand change of approach

Senator John McCain, who is in line to become chairman of the Senate Armed Services Committee, which must approve Hagel's replacement, said he hoped Obama would nominate someone with "strength of character, judgment and independence".

McCain, who like House Speaker John Boehner is a fierce critic of the president, called for changes in his approach to defence.

"A successor will be named in short order, but Secretary Hagel will remain as Defence Secretary until his replacement is confirmed by the United States Senate," a senior Obama administration official said.

Congressional sources said that was almost certainly not going to happen until after January, when Republicans take over in the Senate and control the confirmation process.

"This personnel change must be part of a larger re-thinking of our strategy to confront the threats we face abroad, especially the threat posed by the rise of ISIL (Islamic State)," said Boehner.

"We cannot defeat this enemy without a broad, coordinated, well thought-out effort that has the strong support of the American people. Thus far, this administration has fallen well short," he said in a statement.

A Vietnam War veteran and long-time Republican senator, Hagel, 68, had been criticised by some for failing to clearly articulate policy, including during his confirmation hearing nearly two years ago.

Reuters file photo

Hagel submitted his resignation letter after lengthy discussions with Obama that began in October, officials said.

Obama praised Hagel at a White House event called to announce his departure, saying he had always been candid with his advice and had "always given it to me straight".

Officials said Obama wanted fresh leadership during the final two years of his administration.

Flournoy is possible successor

Top potential candidates to replace Hagel include Michele Flournoy, a former undersecretary of defence, and Ashton Carter, a former deputy secretary of defence, who were rumoured to be contenders for Hagel's job before he was named.

Senator Jack Reed, Democrat of Rhode Island, is another possible contender, although his press secretary said he "has made it very clear that he does not wish to be considered for Secretary of Defence or any other cabinet position".

Hagel, who was the only enlisted combat veteran to serve as defence secretary, ran into a wave of opposition when Obama, a Democrat, nominated him.

Republicans objected partly because Hagel opposed the 2007 'surge' of troops in the Iraq war, which eventually helped defeat al Qaeda and other militants and opened the way for a US troop withdrawal.

He was seen as poorly prepared and hesitant during his confirmation hearing, including refusing to answer 'yes' or 'no' when McCain demanded he judge whether he was wrong to oppose the surge strategy.

Hagel, who became an outspoken critic of the administration of President George W Bush, had also upset many in his party by endorsing Obama in his presidential race against Republican Senator John McCain in 2008.