The Capitol, which is home to the US Congress, was attacked by a pro-Trump mob on 6 January, leaving five dead.
Reporters at the scene say an emergency announcement is playing and a rehearsal for Biden's inauguration has been suspended.
According to the agency, the security is tight as thousands of National Guard members were deployed.
Eyewitnesses reported smoke near the Capitol. The Washington DC fire service said it had responded to a fire nearby which was now extinguished.
An alert circulated to Capitol staff said the complex was on lockdown, with no-one being allowed in or out.