Former Pentagon official Ashton Carter has been nominated by President Barack Obama as new defense secretary replacing Chuck Hagel who resigned last month.

Ashton Carter was previously the Pentagon’s chief arms buyer and an assistant secretary of defense under former President Bill Clinton.

His appointment to the post now requires approval from the Republican-led US Senate.

Ashton Carter received a doctorate in theoretical physics from Oxford.

“We face no shortage of challenges to our national security,” Barack Obama said on December 5, praising Ashton Carter’s more than 30 years of service.

“[He] is going to be critical to all these efforts.”

Ashton Carter called the nomination an “honor and a privilege”, and pledged to Barack Obama his “most candid” strategic and military advice.

Chuck Hagel quit after two years in the job, reportedly under pressure to go.

Following word of his resignation in November, Chuck Hagel called manning the post his life’s “greatest privilege”.

Absent from the nomination announcement at the White House, Chuck Hagel released a statement calling Ashton Carter a “patriot and a leader”, adding he “strongly” supports the nomination.

Vietnam war veteran and former Republican senator Chuck Hagel, 68, will stay on as secretary until Ashton Carter is confirmed.

Chuck Hagel was reportedly sharply critical of the US strategy against Islamic State (ISIS) and in relation to the Syria regime.

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