An "adult man" managed to enter Andrews Air Force base -- which hosts visiting dignitaries in Washington and is where official US planes, including the Air Force One presidential jet, are based -- on Thursday, officials said.

"Everyone takes this very seriously. (Secretary of Defense Lloyd Austin) takes this very seriously," Pentagon spokesman John Kirby said, stressing that the investigation would focus on security measures of the US Air Force around the world.

The unarmed man, whom authorities did not immediately link to any extremist group, was able to illegally board a C-40, the military version of the Boeing 737, before being arrested, the air force added.

The C-40 aircraft stationed at Andrews Air Force Base are used by members of the United States government, senior congressional officials, or senior military officials during their official travels.

The Air Force did not specify how the man had managed to enter the air base, which is closely guarded, or how long he had spent on the plane.

Thousands of National Guard soldiers have been bolstering Washington's security since the assault on Capitol Hill by pro-Trump protesters that left five people dead.

As authorities fear further protests, the Guard are tasked with protecting the Capitol during Donald Trump's second impeachment trial set to start on February 9.

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