US defence secretary Chuck Hagel resigns
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.
Obama accepted the resignation of Hagel, the sole Republican on his national security team, and would formally announce the decision at 11:10am. EST (1610 GMT), officials said.
"A successor will be named in short order, but Secretary Hagel will remain as Defence Secretary until his replacement is confirmed by the United States Senate," an unnamed senior Obama administration official quoted by the Reuters news agency said.
Al Jazeera's Patty Culhane, reporting from Washington, said the move suggests that Obama is paying heed to criticism over the handling of the Islamic State in Iraq and Levant (ISIL) threat in the Middle East.
"Officials are saying that the Pentagon needs an approach different from the one Hagel would bring" if he had stayed, our correspondent said, adding that he was seen as "timid and a bit confused".
Questions on Syria
Hagel had raised questions about Obama's strategy towards 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 towards Syrian President Bashar al-Assad.
Obama has insisted that the United States can go after ISIL fighters 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 US defence official cited by Reuters 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 defence, and Ashton Carter, a former deputy secretary of defence, who were rumoured to be contenders for Hagel's job before he was named.
Senator Jack Reed, Democrat of Rhode Island, is another possible contender.
Al Jazeera
