US House Speaker Nancy Pelosi called Thursday for President Donald Trump to be urgently removed from power, branding him "a very dangerous person who should not continue in office."

"This is an emergency of the highest magnitude," the top-ranking Democrat in Congress said as she urged Vice President Mike Pence and Trump's cabinet to invoke the 25th amendment, one day after a pro-Trump mob stormed the Capitol with his encouragement in a bid to overturn the presidential election results.

If the 25th amendment, allowing for a cabinet majority to remove a president deemed unable to discharge his duties, is not invoked Congress is prepared to move forward with impeachment, she said.

"I don't think it'll take long to get an answer from the vice president," Pelosi said, adding that she and top Senate Democrat Chuck Schumer had "made our interest in this known, so we'll see what they come back with. But they have to answer for it."

She put the blame for what she described as an "attempted coup" squarely on Trump, declaring he had "incited an armed insurrection against America."

"A threshold was crossed of such magnitude that there is no way that this president should be allowed to make any decision, to rouse any troops of his Trumpites. What's next, what happens today?" she said. 

"We do know that he must be contained."

She praised some "courageous" Republicans for finally speaking out against Trump.

But with questions being asked about the security failures in Wednesday's violence she singled out acting defense secretary Christopher Miller, saying he "certainly has to answer for where the National Guard was yesterday."

Trump, she continued, is a "complete tool" of Russian leader Vladimir Putin. 

And simply waiting until his term as president expires on January 20 is not enough, Pelosi said. "In the next 13 days, this dangerous man can do further harm to our country."