US House of Representatives Speaker Nancy Pelosi has accused America’s highest law officer of committing a crime by lying to Congress over his handling of the Mueller report.

The accusation comes as House Judiciary Committee chairman Jerrold Nadler threatened to hold Attorney-General William Barr in contempt of Congress if he did not provide an un-redacted version of Mr Mueller’s findings on the Russia inquiry. 

“He lied to Congress – if anybody else did that, it would be considered a crime,” Ms Pelosi told reporters at a news conference, saying neither the attorney general nor the Republican president is above the law.

Mr Barr failed to attend a House Judiciary hearing on Friday (Australian time), despite testifying before the Senate Judiciary Committee the day before.

“What is deadly serious about it is the Attorney General of the United States of America was not telling the truth to the Congress of the United States. That’s a crime,” Ms Pelosi said.

Speaker Nancy Pelosi blasts Attorney General William Barr following his testimony before the Senate Judiciary Committee: "The attorney general of the United States of America was not telling the truth to the Congress of the United States. That's a crime" https://t.co/2wOHzsAqNU pic.twitter.com/B9dQe49Uu8

— ABC News Politics (@ABCPolitics) May 2, 2019

The Justice Department shot back at Ms Pelosi, shortly after her comments were made.

“The baseless attack on the Attorney General is reckless, irresponsible and false,” Justice Department spokeswoman Kerri Kupec 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.

Ms Pelosi called for Mr Mueller to testify, saying in a Twitter post: “Attorney General Barr misled the public and owes the American people answers”.

Democratic House Oversight Committee Chairman Elijah Cummings echoed her views.

“Barr misled Congress and the American people to protect the President,” he tweeted. “There must be consequences. We must see the letter, get the full report and docs, and hear directly from Mueller.”

A March 27 letter from the special counsel revealed Mr Barr’s initial public summary did not “fully capture the context, nature and substance of this Office’s work”.

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 Nadler over the hearing’s format. Mr Nadler said he will move forward with a contempt citation as soon as Monday.

“We will make one more good faith attempt to negotiate and to get the access to the report that we need, and then if we don’t get that, we will proceed to hold the attorney general in contempt,” Nadler told reporters on Thursday after a 15-minute committee session held in place of Barr’s appearance.

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

Mr Nadler’s comments indicated that the Democrats, who control the House, are prepared to escalate a showdown with the Trump administration, which has resisted their demands for documents and information on a wide range of topics, from Mr Trump’s taxes to his businesses.

Several Democratic lawmakers have called on Mr Barr to resign.

“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 Nadler said Mr Trump wants to prevent Congress from providing any check on his conduct.

“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 said.

Mr Barr spent four hours before a Republican-led Senate committee on Wednesday defending his handling of the Mueller 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.

-with AAP