The rehearsal for US president-elect Joe Biden’s inauguration has come to an abrupt end after security officials ordered an evacuation of the US Capitol West Front.

Capitol Police locked down the building for more than an hour on Monday (local time) after witnesses reported seeing smoke rising nearby.

It was later discovered that a fire in a homeless camp less than two kilometres from the Capitol had sparked the security scare that suspended the rehearsal for Mr Biden’s inauguration.

Those who had gathered for a walk-through, including a military band, were told to head indoors towards a secure location inside the Capitol complex.

People involved in the rehearsal said security officials yelled “this is not a drill”.

“Members and staff were advised to shelter in place while the incident is being investigated,” Capitol Police said.

The Washington fire department said a homeless person’s tent located beneath a freeway had caught fire and was “quickly extinguished”.

The site of a fire in a homeless camp under a bridge. Photo: AAP

“Occupant indicated she was using propane, which may explain report of ‘explosion’,” the department said.

The US Secret Service said the fire had posed “no threat to the public”.

The homeless woman suffered minor injuries but declined to be taken to hospital. No one else was hurt in the incident.

The Capitol complex and surrounding areas have been locked down after the deadly January 6 insurrection by pro-Trump rioters.

Heightened security ahead of Inauguration Day

US defence officials have expressed concern about a potential insider attack or other threat from service members involved in securing Mr Biden’s inauguration on Wednesday.

This had prompted the FBI to vet all of the 25,000 National Guard troops coming into Washington for the event.

Army Secretary Ryan McCarthy said officials are conscious of the potential threat, and he warned commanders to be on the lookout for any problems within their ranks as the inauguration approaches.

Heavily armed police officers responded to a security scare at the US Capitol. Photo: AAP

He said Guard members were being trained to identify potential insider threats.

About 25,000 members of the National Guard are streaming into Washington from across the US – at least two and a half times as many as for previous inaugurals.

While the military routinely reviews service members for extremist connections, the FBI screening is in addition to any previous monitoring.

Multiple officials said the process began as the first Guard troops began arriving in Washington DC more than a week ago. They said it would be complete by Wednesday.

Such FBI vetting is likely to involve running peoples’ names through the bureau’s databases and watchlists.

Alarming notifications could include involvement in prior investigations or terrorism-related concerns, said David Gomez, a former FBI national security supervisor in Seattle.

-with AAP