REPUBLIKA.CO.ID, WASHINGTON -- US Defense Secretary Chuck Hagel resigned on Monday, the first major change to President Barack Obama's Cabinet since his Democrats were routed in midterm elections three weeks ago.

Obama announced the resignation at a White House event with Hagel at his side. Hagel will remain in the job until a successor is in place. Hagel was appointed less than two years ago as Obama pushed his signature program of winding up wars in Afghanistan and Iraq, a process that is being upended this year with US re-engagement in Iraq and greater military cooperation with Kabul.

The former Republican senator, 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.