US Attorney-General William Barr has declined to attend a House Judiciary Committee hearing on his handling of Special Counsel Robert Mueller’s Russia investigation report.

The defiance of Congress has prompted panel chair Jerrold Nadler to accuse Mr Barr of being afraid to testify.

“Barr has just informed us he will not attend tomorrow’s hearing,” Nadler, a Democrat, said on Wednesday (US time) after a contentious earlier Senate session where Mr Barr defended his treatment of the report.

Mr Nadler also said the Justice Department had not complied with his subpoena for Mr Mueller’s full report on Russian meddling in the 2016 election and underlying evidence from the probe.

Committee Democrats vowed to issue a subpoena to force Mr Barr to testify, while Mr Nadler hoped the Attorney-General would reconsider and show up when the hearing convened at 9am on Thursday local time.

“We plan on subpoenaing him if he decides not to show up. He can run but he can’t hide,” Democratic Representative Hakeem Jeffries said.

A Justice Department spokeswoman said the panel had placed “unprecedented and unnecessary” conditions on Mr Barr’s testimony and called the questions posed by committee staff inappropriate.

“The Attorney-General remains happy to engage directly with members,” the statement said.

Earlier, in his first public testimony since releasing a redacted version of the 448-page report on April 18, Democrats hounded Mr Barr – a Trump appointed – with questions about the way he characterised the conclusions of Mr Mueller’s report.

Mr Barr’s decision not to testify for a second day surfaced hours after the committee adopted a more aggressive format that would subject him to an extra hour of questions from politicians, as well as those from Democratic and Republican lawmakers.

Mr Nadler insisted it was not the administration’s place to determine the format for the hearing.

“He is terrified of having to face a skilled attorney,” the chairman said of Mr Barr.

“Given how dishonest he has been … I can understand why he’s afraid of facing more effective examination.”

The House Judiciary chairman also said he would consider a contempt citation against Mr Barr, if the Justice Department continued to defy the subpoena for the unredacted report.

Democrats contend Mr Barr misrepresented the contents of the Mueller report in a March 24 summary and subsequent news conference.

They also allege he may have provided misleading testimony to Congress about criticism he received from Mueller.

However, Mr Barr told Senate Judiciary Committee Democrats why he concluded that Mr Trump had not unlawfully sought to obstruct the 22-month investigation.

“I don’t think the government had a prosecutable case,” Mr Barr, the top US law enforcement official and a Trump appointee, testified.

Mr Barr also defended the way he dealt with the report’s release, redactions made by the Justice Department, and his ultimate conclusion that Mr Trump did not obstruct justice.

Mr Barr was asked specifically about the report’s finding that in June 2017 MR Trump directed White House counsel Don McGahn to tell the Justice Department’s No. 2 official, Rod Rosenstein, that Mr Mueller had conflicts of interest and must be removed.

Mr McGahn did not carry out the order. Mr Rosenstein had appointed Mr Mueller the prior month.

Mr Barr seemed to minimise the incident and said Mr Trump believed “he never outright directed the firing of Mueller.”

“We did not think in this case that the government could show corrupt intent,” Mr Barr said.

Mr Barr was also critical of Mueller for not reaching a conclusion himself on whether Trump obstructed the probe.

“I think that if he felt that he shouldn’t go down the path of making a traditional prosecutorial decision, then he shouldn’t have investigated,” Mr Barr said.

-with AAP