The White House says National Guard troops are heading to the US Capitol to help end a violent occupation by protesters, who have breached the premises.
The US Capitol is in lockdown after protesters broke through barricades, amid violent clashes between President Donald Trump's supporters and Washington DC police.
Both chambers of Congress abruptly recessed as they were debating the Electoral College vote that gave Joe Biden the presidency.
Press secretary Kayleigh McEnany tweeted: "At President @realDonaldTrump's direction, the National Guard is on the way along with other federal protective services."
She added: "We reiterate President Trump's call against violence and to remain peaceful."
NATO Secretary General Jens Stoltenberg has condemned the developments in Washington on social media.
"Shocking scenes in Washington DC. The outcome of this democratic election must be respected," he wrote on Twitter.
Cover image: Protestors in Washington (Picture: PA).