WASHINGTON, Jan 18 (Reuters) - 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 for potential security concerns.

After the Jan. 6 Capitol assault by supporters of 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."

The Washington Post reported Monday the FBI in an intelligence report warned law enforcement agencies that far-right extremists had discussed posing as National Guard members in Washington.

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The Post added the report did not identify any specific plots to attack the inauguration events.

The U.S. Army said on Tuesday it was is 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.

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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. More than a dozen subway stations have been closed through Wednesday's inauguration.

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.

(Reporting by David Shepardson; additional reporting by Doina Chiacu Editing by Bill Berkrot)