In a surprise development, US Defense Secretary Chuck Hagel resigned Monday reportedly under pressure amid criticism of President Barack Obama's national security team over how to deal with the threat posed by the Islamic State militant group.

 

 

 Announcing Hagel's resignation from White House's State Dining Room, Obama called Hagel "an exemplary defense secretary" and "a great friend of mine" who helped steer the military amid a time of great transition  at the Pentagon.

 

 Hagel, the sole Republican on his national security team, called serving in the post "the greatest privilege of my life." He took up the job in February 2013 amid great opposition from his own Republican Party.

 

 However, the New York Times which first reported the resignation cited administration officials as saying Obama made the decision to remove  Hagel, last Friday after a series of meetings between the two men over the past two weeks.

 

 The officials, the influential US daily said, characterised the decision as a recognition that the threat from the Islamic State will require different skills from those that Hagel was brought in to employ.

 

 Hagel, a combat veteran who was skeptical about the Iraq war, came in to  manage the Afghanistan combat withdrawal and the shrinking Pentagon  budget in the era of budget sequestrations, it noted.

 

 NBC News cited unnamed senior defence officials as saying Monday that Hagel was forced to resign as the White House had lost confidence in the former Repunlican senator.

 

 "He wasn't up to the job," one official was quoted as saying.

 

 NBC News also noted that Hagel "was originally brought to the job to wind down the wars in Iraq and Afghanistan."

 

 "Rather than winding down two wars, we're winding up," it said citing a  source close to Hagel and top Pentagon officials.

 

 Obama did not name a successor Monday, but Hagel will stay in the job until his replacement is confirmed.

 

 The short list to replace Hagel includes former under secretary of defense for policy Michale Flournoy, former deputy secretary of defense Ash Carter and Rhode Island Senator Jack Reed, a former paratrooper, CNN  reported citing a White House official.