Pre-riot speech 'totally appropriate': Trump

US President Donald Trump, facing impeachment on a charge of "incitement of insurrection", has denied responsibility for his supporters' violent invasion of the US Capitol last week and said his remarks before the siege were appropriate.

The Republican president told reporters that his speech before last Wednesday's assault - in which he urged supporters to march on the Capitol and fight - had been analyzed by unnamed others, who he said believed it was "totally appropriate."

"If you read my speech ... what I said was totally appropriate," he told reporters at Joint Base Andrews when asked about any personal responsibility he had regarding the Jan. 6 attack when his supporters stormed the Capitol with members of Congress and his own Vice President Mike Pence inside.

"They've analyzed my speech and my words and my final paragraph, my final sentence and everybody ... thought it was totally appropriate," he said before heading to Alamo, Texas to visit and sign his signature wall on the border with Mexico.

Democrats in the US House of Representatives plan to impeach Trump on Wednesday (local time) unless he steps down or is removed before then, which would make him the only US president ever to be impeached twice.

Trump's remarks on Tuesday were his first in public since last Wednesday, although he released a video on Thursday in which he condemned the violence but did not concede the election.

Democrats said Trump's defence of his words and actions underscored the urgency of removing him from office.

"Trump’s lack of contrition today makes clear that the 25th Am (Amendment) must be invoked to remove him," Daniel Goldman, majority counsel in the first House impeachment inquiry, tweeted. "Impeachment should proceed apace and every MOC must think hard about whether s/he supports Trump’s efforts to overturn the election and incite an insurrection on the Capitol."

Trump did not answer a shouted question before leaving the White House about whether he was responsible for the violence at the Capitol, which led to the deaths of six people.

Instead, he blasted Democratic leaders for moving forward on impeachment, warning it posed "a tremendous danger" to the country.

Conservative former presidential candidate Evan McMullin said those remarks amounted to threats of further violence.

"Trump's threats of more violence by warning of 'tremendous anger' and 'tremendous danger' in response to potential impeachment, a vital mechanism of our democracy, are further evidence of its justice and necessity," he said on Twitter.