Physicist Ashton Carter likely to be nominated to be US defence secretary

US President Barack Obama is expected to nominate Ashton Carter, a physicist with extensive experience in the Pentagon's leadership, as secretary of defence to replace Chuck Hagel, according to officials.

If confirmed by the Senate, Carter would lead the Pentagon as it expands military operations against Islamic State militants in Iraq and Syria and deals with a resurgent Russia and renewed violence in Afghanistan. The military will also rebalance forces in Asia, even as it faces budget constraints and a cutback in troop levels.

Carter, 60, could prove more aggressive than the self-effacing Hagel in defending the administration's policies in public, and at pushing back in private against White House attempts to keep tight limits on military operations. Hagel had announced his resignation on November 24 under pressure from the White House and amid disagreements between the Pentagon and the White House over the president's strategy in the latest war in the Middle East.

Hagel will stay in the job until his replacement is confirmed.

Carter, who has a doctorate in theoretical physics from Oxford, never served in uniform. But the military establishment regarded him as a bold thinker when he served in the Obama administration as deputy defence secretary, the No 2 post, and as chief weapons buyer, the No 3 post. He resigned a year ago.

He was confirmed by unanimous Senate votes for his two previous Pentagon posts and is unlikely to face strong opposition this time.

White House officials did not dispute that Carter was the leading contender. But the White House was not prepared to announce the president's choice and was still deliberating on timing. Among the factors playing into the decision was the vetting process, which was not yet complete, one senior administration official said, speaking on condition of anonymity because the decision is not yet final.