US Defence Secretary Chuck Hagel has resigned from his post after serving for less than two years at the Pentagon.
US President Barack Obama's third Pentagon chief and a former Republican senator, Hagel was appointed in February 2013.
He replaced Leon Panetta as defence secretary in Obama's second term.
Obama said: "When I asked Chuck to serve as secretary of defence, we were entering a significant period of transition.
"Over nearly two years, Chuck has been an exemplary defence secretary.
"Thanks to Chuck, our military is on a firmer footing engaged in these missions and looking ahead to the future."
In a statement to US Armed Forces, Hagel said: "I want you to know that I am immensely proud of what we have accomplished together.
"We have prepared ourselves, our allies and the Afghan National Security Forces for a successful transition in Afghanistan.
"We have taken the fight to ISIL and, with our Iraqi and coalition partners, have blunted the momentum of this barbaric enemy.
"And we have bolstered enduring alliances and strengthened emerging partnerships, all the while setting in motion important reforms that will prepare this institution for the challenges facing us in the decades to come."
While the White House insists the resignation was a mutual decision, sources said Hagel was forced to step down as Obama wanted new leadership during the final two years of his tenure.
According to reports, Hagel was unsettled in his national security team, and also had differences with the administration on a number of issues, including its strategy against Islamic State and the Syrian regime.
The 68-year-old Vietnam War veteran will continue in his position until a successor is nominated and confirmed by US Senate.
Top potential candidates to replace Hagel include the former Undersecretary of Defence Michele Flournoy, and Ashton Carter, a former deputy secretary of defence, and US Senator Jack Reed.
Image: US Defence Secretary Chuck Hagel shakes hands with President Barack Obama at the White House. Photo: courtesy photo of US Department of Defense.