UPDATE, 10:30a ET: Chuck Hagel was fired by President Obama, a U.S. official with close knowledge of the situation told Jennifer Griffin this morning.

Though Hagel tendered his resignation officially, the source told Griffin, "make no mistake, Secretary Hagel was fired."

The official told Griffin that the White House had been planning Hagel's exit for weeks because the president lost confidence in Hagel.

Griffin said an administration official characterized the decision as "post-election politics" and noted that Hagel is the only Republican in Obama's cabinet. 

Defense Secretary Chuck Hagel will step down, a senior administration official confirmed this morning to Fox News.

Fox News national security correspondent Jennifer Griffin reported on "America's Newsroom" that the move had been discussed between President Obama and Hagel in recent weeks. 

The resignation is being described as mutual by the administration, but Griffin reported that Hagel has been under pressure to leave. 

She pointed to recent reports about Hagel standing up to National Security Adviser Susan Rice about the strategy against ISIS. 

Meantime, Fox News' James Rosen identifies some possible replacements as Michèle Flournoy, Ash Carter and Jack Reed.

National security analyst K.T. McFarland weighed in after the news broke, saying she believes Flournoy or Carter would be confirmed easily. 

Stay tuned to Fox News for continuing coverage of the developing story. 

Read more from AP:

WASHINGTON — Defense Secretary Chuck Hagel is stepping down from President Barack Obama's Cabinet, senior administration officials said Monday, following a tenure in which he has struggled to break through the White House's insular foreign policy team.

Hagel is the first senior Obama adviser to leave the administration following the sweeping losses for Obama's party in the midterm elections. It also comes as the president's national security team has been battered by multiple foreign policy crises, include the rise of the Islamic State militants in Iraq and Syria.

A senior defense official said that Hagel submitted his resignation letter to Obama on Monday morning and the president accepted it. Hagel agreed to remain in office until his successor is confirmed by the Senate, the official said.

The official said both Hagel and Obama "determined that it was time for new leadership in the Pentagon," adding that they had been discussing the matter over a period of several weeks.

Obama was to announce Hagel's resignation Monday. The president is not expected to nominate a new Pentagon chief Monday, according to one official.

The officials insisted on anonymity because they were not authorized to discuss the matter by name ahead of Obama's official announcement.

Hagel is a Republican who served as senator from Nebraska and became a critic of U.S. involvement in Iraq. Obama nominated him to succeed Leon Panetta as Defense Secretary in his second term.

Hagel served in the Vietnam War and received two Purple Hearts.