The US National Guard has pulled two soldiers from security duty for Joe Biden's presidential inauguration ceremony, CNN reported Tuesday, as troops were vetted for links to extremist groups.

Citing a Defense Department official, the cable network could not confirm whether such a link had indeed been made to the two troops or whether a specific incident had provoked their expulsion ahead of Wednesday's events.

"Due to operational security, we do not discuss the process nor the outcome of the vetting process for military members supporting the inauguration," the National Guard told CNN.

Washington in recent days has become a city in virtual lockdown, with concrete blockades, barbed wire and thousands of armed troops patrolling downtown streets after the violent assault on the Capitol by supporters of President Donald Trump on January 6.

The FBI had announced it would be conducting background checks on the visiting National Guard troops to further identify potential security risks.

We "want to make sure that we have the right people in the security bubble," General William Walker, head of the National Guard in Washington, told Fox News on Monday.

All told, there will be up to 25,000 National Guard troops deployed in Washington, charged with protecting a mostly inaccessible "red zone" around the Capitol -- where Biden and Vice President-elect Kamala Harris will take their oaths of office -- and the White House.

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