National Guard troops stand guard in Downtown Washington, DC on January 18, 2021 ahead of the inauguration ceremonies for President-elect Joe Biden and Vice President-elect Kamala Harris. (Photo: AFP)

The acting Pentagon chief said on Monday the FBI is assisting the U.S. military in vetting more than 25,000 National Guard troops being deployed to assist in protecting the U.S. Capitol around President-elect Joe Biden's inauguration over potential security concerns.

After the January 6 Capitol assault by supporters of U.S. President Donald Trump that resulted in five deaths and sent lawmakers into hiding, the U.S. government has imposed unprecedented security surrounding the Capitol, including non-scalable fences rimmed with razor wire and a large security zone that the public is barred from.

Acting Secretary of Defense Christopher Miller said in a statement on Monday the vetting is "normal for military support to large security events ... While we have no intelligence indicating an insider threat, we are leaving no stone unturned in securing the capital."

Miller said he appreciated "the support of the FBI in assisting with this task and for each of the more than 25,000 Guardsmen."

Flags are placed on the National Mall ahead of the inauguration of President-elect Joe Biden and Vice President-elect Kamala Harris, Monday, Jan. 18, 2021, in Washington. (Photo: AP)

The U.S. Army said on Tuesday it was working with the FBI to see if any attackers were current service members and with the Secret Service to see if any of the nearly 10,000 National Guard troops securing Biden's inauguration would need additional screening.

Asked why authorities were looking into the background of every National Guard member called in to help secure the area around the swearing in ceremony, Acting Department of Homeland Security Secretary Peter Gaynor said on Monday: "The FBI and others decided this would be a prudent move."

He told Fox News he has not seen any evidence of some sort of inside attack being planned, but authorities wanted to leave "no stone unturned" when it comes to potential threats to the peaceful transfer of power.

Biden's inauguration is going to take place on Wednesday with an attendance of around 1,000 guests, mainly members of Congress and their guests.

For security reasons, areas surrounding the White House and Capitol building are closed to the public. There will also be no parade or inaugural balls.

The US Capitol Building is prepared for the inauguration ceremonies for President-elect Joe Biden as the "Field of Flags" are placed on the ground on the National Mall on January 18, 2021 in Washington, DC. (Photo: AFP)

The U.S. government has for days blocked access to major public parks including Washington's National Mall and closed bridges crossing the Potomac River between Virginia and the District of Columbia in efforts to boost the city's security after the Capitol assault and calls for more violence by Trump supporters. Law enforcement and the military have also essentially locked down downtown D.C.

At least one bus company has halted bus trips to Washington ahead of the inauguration, while Airbnb canceled home-sharing reservations in the Washington, D.C., area for the week of Biden's inauguration. U.S. airlines have also imposed new security precautions for DC-area flights.

In a major break from tradition, Trump will not attend the inauguration, instead, he will have a departure ceremony Wednesday morning at Maryland's Joint Base Andrews before leaving for Florida on Air Force One. Vice President Mike Pence, however, will be attending the inauguration.