President Obama named Ashton Carter as his nominee for secretary of defense during a news conference on Friday, saying that during his time as president, he "relied on [Carter's] expertise and...his judgment." (AP)

President Obama nominated a new defense chief on Friday to replace Chuck Hagel, who is departing the Pentagon as the United States expands its fight against Islamic State militants in the Middle East.

The nominee, Ashton B. Carter, is a physicist and former academic who previously served as the Pentagon’s second-highest ranking official.

In making the announcement, Obama listed the array of challenges including battling the Islamic State militants, confronting cyber-terrorism and using military resources to help control the spread of Ebola.

Hagel, a former Republican senator from Nebraska, announced that he would depart the Pentagon following weeks of speculation about a rift between the Pentagon leadership and the White House. Hagel, a decorated veteran of the Vietnam war, took over in early 2013 charged with reforming the Pentagon budget and ushering the U.S. military out of a dozen years of war.

But senior officials indicated that Obama wanted a change as the U.S. military broadens its operations in Iraq and Syria against the extremist group that captured a wide swath of territory in both countries.

Senior lawmakers have indicated their support for Carter, 60, who as deputy defense secretary oversaw the massive Pentagon budget and its weapons procurement.