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 waiver from Congress, but a handful of Democrats including senators Richard Blumenthal, Tammy Duckworth and Jon Tester have already said they would vote against the waiver.

- Austin choice 'feels off' -

House Democrat Elissa Slotkin, a former CIA analyst, said Tuesday that while she respects Austin, his confirmation would "contravene" the essential principle of civilian control of the military.

"Choosing another recently retired general to serve in a role designed for a civilian just feels off," she tweeted.

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."

He said he would surround himself "with experienced and capable civilian appointees and career civil servants who will enable healthy civil-military relations grounded in meaningful civilian oversight."

After retiring, Austin joined the board of Raytheon, and his connection to one of the Pentagon's largest weapons suppliers could come under scrutiny in his confirmation process.

Biden, who becomes the nation's 46th president on January 20, said Austin's intimate knowledge of the Pentagon makes him "uniquely suited for the challenges we face now, the crisis we face now."

The Democrat worked with Austin in Iraq, when he traveled there as vice president, and said he was impressed by Austin's command of troops.

"He's the definition of duty, honor, country," said Biden. "He's cool under fire, inspiring the same in all those around him."

The men share a personal connection. Biden's son Beau, who died of cancer in 2015, served as a military lawyer on Austin's staff in Iraq.