US House of Representatives Speaker Nancy Pelosi has accused Attorney-General William Barr of committing a crime as congressional Democrats stepped up the pressure on President Donald Trump's administration.

Key points: Nancy Pelosi says Attorney-General William Barr committed a crime by lying

Nancy Pelosi says Attorney-General William Barr committed a crime by lying A Justice Department spokeswoman says Ms Pelosi's allegation was "reckless, irresponsible and false"

A Justice Department spokeswoman says Ms Pelosi's allegation was "reckless, irresponsible and false" House Judiciary Committee chairman threatens to hold Mr Barr in contempt of Congress if he doesn't provide a full, unredacted copy of the Mueller report

Ms Pelosi's accusation came as an influential committee chairman threatened to hold Mr Barr in contempt of Congress.

In a defiant letter, the White House said Mr Trump had the right to instruct advisers not to testify before congressional oversight probes related to the Special Counsel Robert Mueller's Russia inquiry and called Mr Mueller's report on the matter defective.

The statements mark a sharp escalation in the conflict between Mr Trump's administration and Democrats in Congress, who are weighing whether they should try to remove Mr Trump from office using the impeachment process as they demand information on his taxes, his businesses and other topics.

Democrats accused the Republican President and his administration of a launching a growing attack on American democracy.

Ms Pelosi, the top Democrat in Congress, accused Mr Barr of lying about his interactions with Mr Mueller after the special counsel concluded his 22-month investigation into Russia's efforts to interfere in the 2016 US election to boost Mr Trump's candidacy.

"He lied to Congress. And if anybody else did that, it would be considered a crime," the top Democrat in Congress told reporters, referring to Mr Barr's congressional testimony.

"Nobody is above the law. Not the President of the United States, and not the Attorney-General.

"Being the attorney-general does not give you a badge to say whatever you want and it is a fact because you are the attorney-general."

Justice Department spokeswoman Kerri Kupec called Ms Pelosi's allegation "reckless, irresponsible and false".

The exchange came shortly after House Judiciary Committee chairman Jerrold Nadler threatened to hold Mr Barr in contempt of Congress if he did not provide a full, unredacted copy of Mr Mueller's findings. That could potentially lead to fines or imprisonment for the top US law-enforcement official.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 1 m William Barr testified in front of a house committee about the Mueller report.

Mr Barr released Mr Mueller's report on April 18, with some parts blacked out to protect sensitive information.

Ms Pelosi and other leaders in the House, which is controlled by Democrats, previously had cautioned against launching the impeachment process, which would begin in the House but would face long odds of success in the Republican-controlled Senate and could alienate voters ahead of the 2020 presidential election in which Mr Trump is seeking re-election.

But Ms Pelosi said the Trump administration was continuing to ignore congressional subpoenas and noted Congress launched impeachment proceedings against president Richard Nixon in the 1970s after he resisted similar demands. Mr Nixon resigned in 1974.

"I think that the statements being made by the President of the United States, as giving a blanket statement that he's not going to honour any subpoenas, is obstruction of justice," Ms Pelosi said.

White House legal counsel Emmet Flood, in an April 19 letter to Mr Barr obtained by Reuters on Thursday, said Mr Trump's decision to let advisers cooperate with the Mueller probe did not extend to congressional oversight investigations.

"It is one thing for a president to encourage complete cooperation and transparency in a criminal investigation conducted largely within the Executive Branch. It is something else entirely to allow his advisers to appear before Congress," the letter said.

Democrats have accused Mr Barr of misleading Congress by testifying in April that he did not know whether Mr Mueller agreed with his initial characterisation of the report — failing to mention a March 27 letter from the special counsel complaining that Mr Barr's initial public summary did not, "fully capture the context, nature and substance of this Office's work". Several Democrats have called on him to resign.

The House Judiciary Committee chairman has threatened to hold William Barr (pictured) in contempt of Congress. ( AP: Andrew Harnik )

Mr Nadler had set a Wednesday deadline for Mr Barr to hand over the unredacted report and its underlying evidence. Mr Barr cancelled his testimony after clashing with Mr Nadler over the hearing's format. Mr Nadler said he would move forward with a contempt citation as soon as Monday.

"The very system of government of the United States — the system of limited power, the system of not having a president as a dictator — is very much at stake," Mr Nadler told reporters after a 15-minute committee session held in place of Mr Barr's appearance.

Democrats have said they may issue a subpoena to try to force Mr Barr to testify before their committee.

"The failure of Attorney-General Barr to come to the hearing today is simply another step in the administration's growing attack on American democracy and its attack on the right of Congress" as a co-equal branch of government, Mr Nadler said.

Mr Barr spent four hours before a Republican-led Senate committee on Wednesday, defending his handling of Mr Mueller's report on Russia's interference in the election to boost Mr Trump's candidacy and whether the President subsequently tried to obstruct Mr Mueller's probe.

Representative Doug Collins, the House Judiciary Committee's top Republican, said Mr Barr did not appear before that panel because Mr Nadler had insisted on an aggressive format with an extra hour of questioning from its own lawyers, in addition to those from the committee's lawmakers.

"They want it to look like an impeachment hearing because they won't bring impeachment proceedings," Mr Collins said.

The report detailed extensive contacts between Mr Trump's 2016 presidential campaign and Moscow and the campaign's expectation that it would benefit from Russia's actions, which included hacking and propaganda to boost Mr Trump and harm Democratic candidate Hillary Clinton. The report also detailed a series of actions Mr Trump took to try to impede the investigation.

Mr Mueller, a former FBI director, concluded there was insufficient evidence to show a criminal conspiracy and opted not to make a conclusion on whether Mr Trump committed obstruction of justice, but pointedly did not exonerate him.

Mr Barr has said he and Rod Rosenstein, the Justice Department's number two official, then determined there was not enough evidence to charge Mr Trump with obstruction.

Reuters