At a press event in Wilmington, Delaware, Biden described Austin as "the definition of duty, honor, country" as he called on the Senate to make an exception to the law requiring any officer who heads the department be out of military service at least seven years.

Austin, a 67-year-old four-star general, was US commander in Iraq and then head of the US Central Command covering all of the Middle East from 2010 to 2016.

"I believe in the importance of civilian control of the military. So does the secretary designate," said Biden, who worked with Austin in Iraq when he visited the country as vice president.

"I would not be asking for this exception if I did not believe this moment in our history didn't call for it. It does call for it," he added.

"So just as they did for secretary Jim Mattis, I ask the Congress to grant a waiver for Secretary-designate Austin."

The law has been waived only twice before, including in 2017, when President Donald Trump nominated Mattis as defense secretary.

Overcoming that barrier will require a bipartisan vote in Congress, but three Democrats -- Senators Richard Blumenthal, Tammy Duckworth and Jon Tester -- have already said they would oppose the waiver out of concern the post should be led by a civilian.

Austin, who would be the Pentagon's first Black leader, said he would bring military experience to the role along with "a deep appreciation and reverence for the prevailing wisdom of civilian control of our military."

Biden will become the 46th president of the United States on January 20.