WASHINGTON (REUTERS) - Mr Ashton Carter, President Barack Obama's nominee to replace Mr Chuck Hagel as US defence secretary, has undergone a long-planned surgical procedure on his back ahead of his confirmation process in the Senate, the White House said on Friday.

White House spokesman Josh Earnest said in a statement that the procedure was successful but did not give details about the operation.

"Mr Carter decided to go ahead with the previously scheduled procedure so that he could complete it before beginning his confirmation process," Earnest added.

"Mr Carter is resting comfortably with his family and is looking forward to (the) confirmation process and if confirmed, to serving with the men and women of the Defense Department," Mr Earnest said.

Mr Obama formally nominated Mr Carter, 60, for the Pentagon post on Dec 5, choosing him for a job that will entail tackling messy wars in the Middle East and Afghanistan and trying to fend off potentially damaging budget cuts.

If confirmed by the Senate, he would become the fourth defence secretary to serve under Obama.

The confirmation process is expected to begin early next year.

Mr Hagel, who resigned under pressure on Nov 24, had privately expressed frustration with the Obama administration's strategy on Iraq and Syria and with his lack of influence over the policy, which was driven by the White House.