U.S. President Barack Obama has confirmed that Defense Secretary Chuck Hagel was stepping down after less than two years in office.
Speaking at the White House on November 24, Obama said that Hagel had concluded it's an "appropriate time" for him to complete his service.
Flanked by Hagel and Vice President Joe Biden, Obama said that Hagel, 68, had been an “exemplary” defense secretary.
He said Hagel would stay in place at the Pentagon until a successor is confirmed by the Senate.
The former senator and Vietnam war veteran said the Obama administration has put the Defense Department and the United States on a stronger course toward stability and security.
Hagel said he's "immensely proud" of what was accomplished during his tenure.
The United States has strengthened alliances and partnerships overseas while responding successfully to global crisis, Hagel said.
He said the United States has prepared itself, its allies, and Afghanistan's military for a successful transition.
Hagel’s resignation follows criticism from government opponents over the military campaign in Afghanistan and operations targeting Islamic State (IS) militants in Syria and Iraq.
In recent weeks, Hagel reportedly sent a letter to national security adviser Susan Rice in which he said Obama needed to articulate a clearer view of the administration's approach to dealing with Syrian President Bashar Assad. The letter is said to have angered White House officials.
U.S. Republican Senator John McCain said Hagel's 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."
Hagel's "situation was no different," McCain said in a statement on November 24.
Hagel's departure coincides with a period of great uncertainty over the course of the administration's campaign to defeat the IS extremist group, as well as worry over Russia's actions in Ukraine.
John Boehner, who leads Republicans in the U.S. House of Representatives, said Hagel's replacement must be accompanied by a new look at U.S. military policies.
"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 [the Islamic State group]," Boehner said in a statement.
Among the leading contenders to replace Hagel is Michele Flournoy, who served as the Pentagon's policy chief for the first three years during Obama's first term.
Flournoy, who would be the first woman to head the Pentagon, is now chief executive officer of the Center for a New American Security, a think tank that she cofounded.