A congressional staffer carrying a gun was allowed inside the US Capitol complex for several minutes on Thursday before being arrested, even though an X-ray machine at a security checkpoint had picked up the weapon in his bag.

The 57-year-old man told police he had left the handgun in his bag by accident but the incident is sure to raise alarm over ongoing security breaches, almost a year after the deadly January 6 insurrection.

With numerous dignitaries including President Joe Biden due in the Capitol Rotunda for Republican statesman Bob Dole's memorial, the House of Representative's worker was waved through security at the nearby Longworth office block across the road.

The breach was noticed and he was detained four minutes after entering. But the delay was not significantly shorter than the typical duration of mass shootings in the United States.

The US Capitol Police did not respond to a request for confirmation of media reports that the gun was loaded.

But the force issued a statement saying officers "spotted the image of a handgun in a bag on the X-ray screen" after the staffer had passed though security.

"The man was tracked down four minutes later and arrested. The owner of the bag was identified as 57-year-old Jeffrey Allsbrooks, a staffer with the House Chief Administrative Office," said the statement said.

"Allsbrooks told our officers he forgot the gun was in the bag. We are charging him with carrying a pistol without a license. This case remains under investigation and the department is looking into what happened before, during and after those four minutes."

Lockdown 

The agency sent out an alert just before 8:00 am notifying Longworth staff of a security threat and telling them to lock themselves in the nearest office and stay away from all doors and windows, or "seek cover if in a public space."

Staff were prevented from entering or leaving the building during the lockdown, which was lifted after a few minutes.

The arrest came with Biden preparing to pay tribute to former Republican senator and presidential candidate Dole, who is lying in state in the Capitol.

The police have been on high alert since one of their officers died and numerous others were wounded as thousands of supporters of defeated president Donald Trump stormed the Capitol on January 6 in a bid to overturn Biden's election victory.

Security was tested again on April 2 when a man rammed a car into police at a barricade outside the complex, wounding one officer.

And government offices were evacuated when a motorist stopped his pickup truck outside the Library of Congress, across the road from Congress, in August and said he had explosives.