Right time for Pentagon to have new leadership: Chuck Hagel

United States Defence Secretary Chuck Hagel resigned on Monday amid reports of differences with President Barack Obama who apparently wants to install a new leadership at the Pentagon as he enters the last two years of his presidency facing new global challenges like the rise of IS.

Obama himself made the surprise announcement at a White House media availability convened at a very short notice.

Hagel had tendered his resignation a few hours ago. He has been asked to stay till a replacement is announced by Obama and is confirmed by the Senate.

"Over nearly two years, Chuck has been an exemplary defence secretary, providing a steady hand as we modernised our strategy and budget to meet long-term threats while still responding to immediate challenges like IS and Ebola," Obama said in his address to the media at the White House's State Dining Room with Vice President Joe Biden, Hagel and the top Pentagon leadership standing by his side.

"Last month, Chuck came to me to discuss the final quarter of my presidency and determined that having guided the department through this transition, it was an appropriate time for him to complete his service," said the president as he praised his third defence secretary for the last two years of his service.

Robert Gates and Leon Panetta were the earlier two defence secretaries.

Obama said, Hagel has played a key role in the accomplishments of his administration in the defence sector.

"Today, our men and women in uniform are taking the fight against IS in Iraq and Syria, and Chuck helped build the international coalition to ensure that the world is meeting this threat together," he said.

"Today, our forces are helping support the civilian effort against Ebola in West Africa, a reminder, as Chuck likes to say, that America's military is the greatest force for good in the world," Obama said.

Hagel said it was the right time to have a new leadership at the Pentagon.

"As the president noted, I have, today, submitted my resignation as secretary of defence. It's been the greatest privilege of my life to lead and most important, to serve with the men and women of the defence department and support their families. I am immensely proud of what we've accomplished during this time," he said.

"We have prepared ourselves, as the president has noted, our allies and Afghan national security forces for successful transition in Afghanistan. We've bolstered enduring alliances and strengthened emerging partnerships while successfully responding to crises around the world," he said.

"And we've launched important reforms that the president noted, reforms that will prepare this institution, the challenges facing us in decades to come. I believe we have set not only this department, the Department of Defence, but the nation on a stronger course toward security, stability and prosperity. If I didn't believe that, I would not have done this job," Hagel said.

"You should know I did not make this decision lightly. But after much discussion, the president and I agreed that now was the right time for new leadership here at the Pentagon," Hagel said in a message to the Department of Defence.

In a statement, Republican Senator John McCain said Hagel was frustrated with aspects of the administration's national security policy and decision-making process.

"His predecessors have spoken about the excessive micro-management they faced from the White House and how that made it more difficult to do their jobs successfully. Chuck's situation was no different," he said.

"I hope the president will nominate a secretary of defence with the strength of character, judgment, and independence that Bob Gates, Leon Panetta, and Chuck Hagel all exhibited at their best.

"But ultimately, the president needs to realise that the real source of his current failures on national security more often lie with his administration's misguided policies and the role played by his White House in devising and implementing them. That is the real change we need right now," McCain said.

Democratic leader Nancy Pelosi said Hagel as Defence Secretary brought personal experiences to a changing and challenging national security landscape – and strove to shape it.

Senate Majority Leader, Harry Reid, hoped that with the US facing threats to national security around the world, Senate Republicans will work with Democrats to give swift and fair consideration to Obama's next nominee to this critical post.

"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 IS. 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," said House Speaker, John Boehner.