"The USCP is responding to a suspicious vehicle near the Library of Congress," the US Capitol Police said on its Twitter feed.

"This is an active bomb threat investigation."

The Federal Bureau of Investigation says it has also joined the probe.

US media reported that a man in a pickup truck made the bomb threat, and that law enforcement and the FBI have dispatched negotiators to try to engage the suspect.

The Library of Congress's main building has been evacuated, as has a Senate office building, according to the reports.

The nearby Republican National Committee headquarters was also reportedly evacuated.

Both the Senate and House of Representatives are currently on recess, but staffers are working in the Capitol complex.

Tensions have remained high on Capitol Hill more than seven months after the deadly January 6 insurrection, when supporters of then-president Donald Trump stormed the US Capitol, fought with police, and sought to block certification of the presidential election.

In April a man rammed a car into barriers at the US Capitol, killing one Capitol Police officer before the attacker was shot and killed.