Former US president Donald Trump rebuffed his own security's warnings about armed protesters in the January 6 rally crowd and made desperate attempts to join his supporters as they marched to the Capitol, according to dramatic new testimony before the House committee investigating the 2021 insurrection.

Cassidy Hutchinson, a little-known former White House aide, described an angry, defiant president who was trying that day to let armed protesters avoid security screenings at a rally that morning to protest his 2020 election defeat and who later grabbed at the steering wheel of the presidential SUV when the Secret Service refused to let him go to the Capitol.

And when the events at the Capitol spiralled toward violence, with the crowd chanting to “Hang Mike Pence”, she testified on Tuesday that Trump declined to intervene.

Trump “doesn't think they're doing anything wrong”, Hutchinson recalled hearing from her boss, White House chief of staff Mark Meadows.

Hutchinson's explosive, moment-by-moment account of what was happening inside and outside the White House offered a vivid description of a Republican president so unwilling to concede his 2020 election defeat to Democrat Joe Biden that he acted out in rage and refused to stop the siege at the Capitol.

It painted a damning portrait of the chaos at the White House as those around the defeated president splintered into one faction supporting his false claims of voter fraud and another trying unsuccessfully to put an end to the violent attack.

Her testimony, at a surprise hearing announced just 24 hours earlier, was the sole focus at the hearing, the sixth by the committee this month. The account was particularly powerful because of her proximity to power, with Hutchinson describing what she witnessed first-hand and was told by others in the White House.

Hutchinson said that she was told Trump fought a security official for control of the presidential SUV on January 6 and demanded to be taken the Capitol as the insurrection began, despite being warned earlier that day that some of his supporters were armed.

The former aide said that she was told of the altercation in the SUV immediately afterward by a White House security official, and that Bobby Engel, the head of the detail, was in the room and didn't dispute the account at the time.

Engel had grabbed Trump's arm to prevent him from gaining control of the armored vehicle, she was told, and Trump then used his free hand to lunge at Engel.

That account was quickly disputed on Tuesday, however. Engel, the agent who was driving the presidential SUV, and Trump security official Tony Ornato are willing to testify under oath that no agent was assaulted and Trump never lunged for the steering wheel, a person familiar with the matter said.

The person would not discuss the matter publicly and spoke to The Associated Press on condition of anonymity.

As the events of January 6 unfurled, Hutchinson, then a special assistant to Meadows, described chaos in White House offices and hallways.

Trump's staff — several of whom had been warned of violence beforehand — became increasingly alarmed as rioters at the Capitol overran police and interrupted the certification of Biden's victory.

Trump was less concerned, she said, even as he heard there were cries in the crowd to “Hang Mike Pence!” Hutchinson recalled that Meadows told aides that Trump “thinks Mike deserves it".

The president tweeted during the attack that Pence didn't have the courage to object to Biden's win as he presided over the joint session of Congress.

The young ex-aide was matter-of-fact in most of her answers. But she did say that she was “disgusted” at Trump's tweet about Pence during the siege.

“It was unpatriotic, it was un-American, and you were watching the Capitol building get defaced over a lie," Hutchinson said, adding that, “I still struggle to work through the emotions of that.”

Trump denied much of what Hutchinson said on his social media platform, Truth Social. He called her a “total phony” and “bad news”.

Members of the panel praised Hutchinson's bravery for testifying and said that other witnesses had been intimidated and did not cooperate.

“I want all Americans to know that what Ms. Hutchinson has done today is not easy,” said Wyoming Rep. Liz Cheney, a Republican who led questioning.

Some of Hutchinson's former colleagues, too, defended her account. Mick Mulvaney, who preceded Meadows as Trump's chief of staff, tweeted that he knows Hutchinson and “I don't think she is lying”.

Sarah Matthews, a former Trump press aide who has also cooperated with the committee, called the testimony “damning”.

As she described the scene in the White House after the election, Hutchinson depicted a president flailing in anger and prone to violent outbursts. Some aides sought to rein in his impulses. Some did not.

At one point on January 6, Hutchinson said, White House counsel Pat Cipollone barreled down the hallway and confronted Meadows about rioters breaching the Capitol. Meadows, staring at his phone, told the White House lawyer that Trump didn't want to do anything, she said.

Earlier, Cipollone had worried out loud that “we're going to get charged with every crime imaginable” if Trump went to the Capitol after his speech at the rally, Hutchinson recalled.

Before the crowd left for the Capitol, Hutchinson said she also received an angry call from House Republican leader Kevin McCarthy, who had just heard the president say he was coming. “Don't come up here,” McCarthy told her, before hanging up.

Hutchinson told the panel that Trump had been informed early in the day that some of the protesters outside the White House had weapons. But he responded that the protesters were “not here to hurt me”, Hutchinson said.

She quoted Trump as directing his staff, in profane terms, to take away the metal-detecting magnetometers that he thought would slow down supporters who were gathering for his speech on the Ellipse, in back of the White House.

In a clip of an earlier interview with the committee, she recalled the president saying words to the effect of: “I don't f-in' care that they have weapons.”

As a White House insider, Hutchinson told stories of a raging president who was unable to acknowledge his defeat. At the beginning of December, she said, she heard noise inside the White House around the time an Associated Press article was published in which Attorney General William Barr said the Justice Department had not found evidence of voter fraud that could have changed the election's outcome.

She said she entered a room to find ketchup dripping down a wall and broken porcelain. The president, it turned out, had thrown his lunch at the wall in disgust over the article. Trump denied it in his social media posts.

In the days before the attack, Hutchinson said she was “scared, and nervous for what could happen” on January 6 after having conversations with Trump lawyer Rudy Giuliani, Meadows and others.

Meadows told Hutchinson that “things might get real, real bad”, she said. Giuliani told her it was going to be “a great day” and “we're going to the Capitol.”

Eventually, both men would seek pardons related to what happened that day, Hutchinson said. A person familiar with the matter denied that Meadows had ever sought a pardon. The person spoke on condition of anonymity.