"The activity that we've seen recently in terms of potential racists or extremist behavior within our ranks is in my view absolutely unacceptable," said Austin, a former general who is set to become the first African American to head the Pentagon.

In a hearing of the Senate Armed Forces Committee, Austin promised "to rid our ranks of racists and extremists, and to create a climate where everyone fit and willing has the opportunity to serve this country with dignity."

His comments came as 12 members of the National Guard force deployed to protect Biden's inauguration on Wednesday were removed during a sweep of background checks to root out any members with potential links to extremist groups.

"The job of the Department of Defense is to keep America safe from our enemies. But we can't do that if some of those enemies lie within our own ranks," said Austin, whose nomination has to be confirmed by the Senate.

When asked to name the main threat to the United States, the 67-year-old cited the coronavirus pandemic, followed by China.

"It's killed over 400,000 of our American citizens. That's just an incredible, incredible loss of life," he said, offering his department's help to combat the virus.

But he noted that "China is our most challenging, our most significant challenge going forward."

He said that under his leadership, the Pentagon would "make sure that we are prepared to meet any challenge and that we continue to present a credible deterrent to China or any other aggressor who would want to take us on, and convince them that would be a really bad idea."

Austin remained vague about specific projects he would undertake as head of the US military, but in his written responses to the senators, he said he planned to re-examine outgoing President Donald Trump's proposals to pull US troops out of Germany and Somalia.

By contrast he said he supported a withdrawal from Afghanistan. "I certainly would like to see this conflict end with a negotiated settlement," he said, while insisting that the US remain focused on "counter-terrorism issues" in the war-torn country.

Asked about the threat from Iran, with whom Biden has said he wants to resume dialogue, he said Tehran remained a "destabilizing element in the region" and that it would be dangerous if the Iranian regime had access to nuclear weapons.