Austin, who is the first Black person to serve as Pentagon chief, has given all units 60 days to hold a day of talks on extremism, his spokesperson John Kirby said.

The aim is to teach troops about the dangers of extremism and misinformation, but also to hear their views, Kirby added.

Over the past decade, several successive US administrations have acted slowly, if at all, on FBI and Department of Homeland Security warnings of white supremacists joining the police and military.

The Pentagon has never published figures on how many extremists it has booted from service.

The storming of the Capitol building was a "wake-up call," Kirby said, adding the Pentagon does not yet understand the scale of the problem that "we haven't solved."

The Pentagon pulled 12 National Guard troops from duty ahead of President Joe Biden's inauguration as part of an extremism screening for the January 20 event.

Biden on Tuesday paid a final tribute at the Capitol to Brian Sicknick, the policeman who died of injuries he suffered in an attack by former president Donald Trump's supporters as they tried to overturn Biden's election win.