WASHINGTON - US Defence Secretary Chuck Hagel has resigned under pressure amid criticism of President Barack Obama’s national security team on a series of global issues, including the threat posed by the militant group known as the Islamic State.

Obama announced the resignation at a White House ceremony with Hagel at his side. Hagel will remain in the job until a successor is confirmed by the Senate.

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

The former Republican senator, who had struggled to improve his ties with Congress after a contentious 2013 confirmation hearing, submitted his resignation letter after lengthy discussions with Obama that began in October, officials said.

During a speech at the White House, President Barack Obama called Hagel an exemplary defence secretary and his great friend who guided the military amid a time of great transition at the Pentagon.

“When it’s mattered most behind closed doors, in the Oval Office, you’ve always given it to me straight. For that, I will always be grateful,” Obama said, noting Hagel’s willingness to take the job despite hailing from the Republican Party. Hagel called serving in the post the greatest privilege of his life.

Behind the scenes, senior defence officials were cited in media reports as saying Monday that Hagel was forced to resign. They said the White House lost confidence in the former Nebraska senator to carry out his role at the Pentagon. “He wasn’t up to the job,” one official was quoted as saying.

Another administration official said Hagel had been discussing a departure from the White House for several weeks.

President Obama said: “Over the past two years, Secretary Hagel helped manage an intense period of transition for the United States armed forces, including the drawdown in Afghanistan, the need to prepare our forces for future missions, and tough fiscal choices to keep our military strong and ready. Over nearly two years, Secretary Hagel has been a steady hand, guiding our military through this transition, and helping us respond to challenges from ISIS to Ebola.”

In October, he began speaking with the President about leaving the administration given the natural post-midterm transition time.

Hagel was originally brought to the job to slow the wars in Iraq and Afghanistan, but, as the fight against the Islamic State ramped up, he was not found fit for the post. A successor will be named in short order, an official said, but Hagel will stay in the job until his replacement is confirmed. Possible nominees include: Rhode Island Senator Jack Reed, former Undersecretary of Defence Michele Flournoy (who would be the first female defence secretary) and former Deputy Secretary of Defence Ashton Carter.