Ashton Carter took office Tuesday as the 25th US secretary of defence after being sworn in by Vice President Joe Biden at the White House.
Carter's nomination was approved 93-5 last week by the Senate.
Carter, 60, a physicist by training, was deputy secretary of defence from 2011 to 2013 under Hagel and his predecessor Leon Panetta. In the post, Carter was the Pentagon's chief operating officer, overseeing more than 600 billion dollars and 2.4 million military and civilian personnel.
After the end of the Cold War, he was directly involved in the efforts to secure Soviet-era nuclear weapons and was present at the signing in Hungary of the 1994 Budapest Memorandum on Security Assurances.