U.S. President Barack Obama is expected to nominate former Deputy Defense Secretary Ashton Carter to be his next defense chief after Chuck Hagel offered to quit last week, CNN and other U.S. media outlets reported Tuesday.

Carter has had three stints at the Pentagon, most recently as deputy secretary from 2011-2013. Though he has no military experience, Carter has a good reputation and his nomination is unlikely to face strong opposition from the Senate.

His two other previous stints at the Pentagon are as under secretary of defense for acquisition, technology and logistics from 2009-2011, and assistant secretary for international security policy from 1993-1996. As assistant secretary, he also participated in negotiations to defuse the first North Korean nuclear crisis.

Carter's nominations for the Pentagon's No. 2 and No. 3 jobs were all approved unanimously at the time.

He received bachelor's degrees in physics and in medieval history from Yale University, and his doctorate in theoretical physics from the University of Oxford. At Harvard's Kennedy School, he was professor and chairman of the International Relations, Science, and Security faculty.

Carter visited Seoul in March last year when North Korea was ratcheting up tensions with daily threats of nuclear war with the South and the U.S. While in Seoul, Carter revealed a U.S. plan to fly nuclear-capable B-52 bombers over South Korea on a training mission.

The highly unusual disclosure was seen as a show of force and warning to the North. (Yonhap)