Hagel resigns as Defense Secretary, official says

Obama was to announce Hagel's resignation at a White House event at 11:10 a.m. on Monday.

Hagel, 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.

"A successor will be named in short order, but Secretary Hagel will remain as Defense Secretary until his replacement is confirmed by the United States Senate," a senior Obama administration official said.

Hagel had raised questions about Obama's strategy toward Syria in a two-page internal policy memo he wrote that leaked. In it, he warned that Obama's policy was in jeopardy due to its failure to clarify its intentions toward Syrian President Bashar al-Assad.

Obama has insisted that the United States can go after Islamic State militants without addressing Assad, who the United States would like to leave power.

Officials said Obama wanted fresh leadership during the final two years of his administration.

“What I can tell you is there are no policy differences in the background of this decision,” a senior U.S. defense official said.

“The secretary is not resigning in protest and he’s not being ‘fired'," the official said.

Top potential candidates to replace Hagel include Michele Flournoy, a former under secretary of defense, and Ashton Carter, a former deputy secretary of defense, who were rumored to be contenders for Hagel's job before he was named.

Senator Jack Reed, Democrat of Rhode Island, is another possible contender.