President Barack Obama on Friday named Ashton Carter, a highly regarded technocrat, to lead the Pentagon as the US military embarks on the latest phase in its 13-year war against Islamic extremists.

Obama praised the 60-year-old Carter, saying that as US defense secretary he would bring to the job "a unique blend of strategic perspective and technical know-how."

"All of which means on day one, he will hit the ground running," Obama said, making the announcement from the White House.

Carter, who is expected to be confirmed by the Senate, was named to replace Chuck Hagel, who resigned under pressure last week, criticized by some as too passive in the face of a rapidly changing security situation.

The US military that Carter is expected to inherit finds itself in an air war against Islamic extremists in Iraq and Syria, a sharp reversal of course for an administration that had sought to bring home troops after 13 years of fighting.

He must also oversee the withdrawal of US forces from Afghanistan, as well as challenges posed by Iran's nuclear program and China's growing military might.

Carter said he had accepted Obama's offer "because of the seriousness of the strategic challenges we face -- but also the bright opportunities that exist for America if we can come together to grab hold of them."

Hagel called Carter after the announcement and promised a smooth transition. In a statement issued as he embarked on his last foreign trip as secretary, he said he strongly supported Carter's nomination.

Hagel, a Republican, has rejected accounts that he was forced out and said it was a mutual agreement with the president.

"But I think you have to know when to leave, too," he added.

- Hard-charging style -

A policy wonk with degrees in Medieval history and theoretical physics, Carter has gained a reputation as an expert on hi-tech weapons and military budgets, portraying himself as a reformer intent on making the vast Pentagon bureaucracy more efficient.

He served as deputy secretary of defense from 2011 to 2013 under former defense secretary Leon Panetta, and in other top Pentagon posts over the years.

But he has less experience overseeing war strategy and has never served in uniform -- unlike Hagel, a former army sergeant who was wounded in the Vietnam War.

He also has a reputation for a hard-charging, blunt-spoken style that could collide with what his predecessors have described as the White House's "micro-management" tendencies.

"There are big uncertainties with respect to Carter's ability to shape US defense policy under an administration this centralized," said Stephen Biddle, a professor at George Washington University who got to know Carter at Harvard.

But, Biddle told AFP, "it's hard to imagine someone better prepared for this job."

"He's widely respected among Pentagon civilians and the uniformed military. He's a capable, experienced manager. He's breathtakingly smart," Biddle said.

The White House on Thursday sought to play down the tension with the Pentagon during Obama's tenure, saying such friction is "not unique to this administration."

However, spokesman Josh Earnest stressed that the next defense secretary will "understand that the president of the United States is the commander in chief and sits atop the chain of command."

In recent years, Carter's views have not always meshed perfectly with the Obama administration.

On Iraq for example, he favored keeping US troops in the country instead of withdrawing forces in 2011, and he has repeatedly warned of the dangers of scaling back defense spending.

Republicans said they would likely confirm Carter, but promised tough questions about the administration's strategy against Islamic State, the extremist group in control of parts of Syria and Iraq.