Image copyright Getty Images Image caption Ashton Carter received a doctorate in theoretical physics from Oxford University

US President Barack Obama is expected to nominate former Pentagon official Ashton Carter as new defence secretary.

Administration officials told the BBC that Mr Carter heads the White House shortlist to replace outgoing Chuck Hagel who resigned last month.

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

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

A Rhodes scholar, he received a doctorate in theoretical physics from Oxford University.

Image copyright AP Image caption Chuck Hagel (right) served as the defence chief for two years under Mr Obama

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

Following word of his resignation in November, Mr Hagel called manning the post his life's "greatest privilege".

The Vietnam war veteran and former Republican senator, 68, will stay on as secretary until Mr Carter is confirmed.

Mr Hagel was reportedly sharply critical of the US strategy against Islamic State and in relation to the Syria regime.

The US president said he was "extraordinary lucky" to have had Mr Hagel by his side as defence secretary and praised him for forging a strong bond with US troops.