U.S. Attorney General William Barr testified Tuesday before a U.S. congressional panel that he should be ready to release a redacted version of special counsel Robert Mueller's report into Russian interference in the 2016 presidential election "within a week."

Barr was appearing before a House of Representatives appropriations subcommittee meant to explore the Trump administration's $29 billion US fiscal 2020 budget request for the Justice Department, but the expectation that Barr would face questions about Mueller's report from the Democrat-led panel was borne out early in the proceedings.

"Within a week I will be in position to release that report to the public and then I will engage with the chairmen of both [House and Senate] judiciary committees about that report, about any further requests that they have," said Barr.

Barr said the redactions would be colour-coded, with explanations, earning the derision of Democratic congressman David Cicilline. Barr also said that the congressional committees would not be presented with an unredacted version.

“Color-coded redactions.” Just release the report already. —@davidcicilline

On March 22, Mueller turned over to Barr his confidential report into whether Donald Trump and his campaign and transition teams may have conspired with Russia during his 2016 presidential campaign and whether Trump later obstructed inquiries into the matter.

On March 24, Barr released a four-page letter to Congress laying out what he said were Mueller's main findings. Trump has used Barr's statements to claim complete exoneration.

Barr testified on Tuesday that Mueller was offered the opportunity to review that summary but declined. He said the redactions of the full report were necessary so that intelligence sources and methods or ongoing prosecutions would not be compromised.

Russia weighs in

Barr was vague in response to a question on whether the White House was briefed on the contents beyond his summary.

"I said what I'm going to say about the report today … and I'm not going to say anything more about it until the report is out and everyone has a chance to look at it," he said.

Russian President Vladimir Putin weighed in on the probe Tuesday for the first time since Mueller finished it. He mocked the investigation of Kremlin interference, saying "a mountain gave birth to a mouse."

Putin sought to cast the 22-month investigation as a failure and disregarded the special counsel's exposure of a Russian operation to put Donald Trump in the White House.

"It was clear for us from the start that it would end like this," the Russian leader said.

Russian President Vladimir Putin mocked the Mueller investigation Tuesday, saying 'a mountain gave birth to a mouse.' (Dmitri Lovetsky/Associated Press)

However, Mueller did uncover evidence of a Kremlin operation to interfere with the 2016 vote. He charged 12 Russian military intelligence officers with breaking into Democratic Party computers and the email accounts of officials with Hillary Clinton's campaign.

Another indictment detailed Russia's use of phony social media accounts to spread divisive rhetoric and to undermine the U.S. political system.

Barr's conclusions 'suspicious': Democrat

Democrats have called for the report to be released in full. They have said they want to review the underlying evidence after Barr in his letter said that Mueller's investigation did not establish collusion with Russia.

Many lawmakers were irked that Barr took it upon himself to decide that Trump should not be charged with obstruction of justice after saying Mueller's report laid out evidence "on both sides" of the question without reaching a conclusion.

New York Democrat Nita Lowey said Tuesday she thought it was "extraordinary" that Barr was able to make definitive conclusions in his letter, just two days after Mueller handed in his findings of a 22-month investigation.

"I would argue it's more suspicious than impressive," said Lowey.

The New York Times and the Washington Post have since reported that some investigators on Mueller's team were unhappy with the way Barr described their findings.

New York Democrat Nita Lowhey questioned Barr on why he would 'cherry pick' the Mueller report, which runs hundreds of pages, into a four-page letter released on March 24. (Andrew Harnik/Associated Press)

In fact, some of the evidence against Trump was more damning than Barr's letter indicated, according to the news reports.

Barr said he did not have insight into why some on Mueller's team were upset.

"I suspect that they probably wanted more put out," he said.

Last week, Democrats on the House judiciary committee prepared subpoenas that they plan to issue to the Justice Department if Barr does not agree to release the Mueller report in full.

Barr's opening statement, released to the media on Monday, made no mention of the Mueller report.

House Republicans early on indicated they were keen to explore America's opioid crisis and security on the border.

Barr avoided using the word "wall," but said he was in favour of a "barrier system" to better allow for drug interdiction and combating human trafficking.

Republicans also asked about the FISA (Foreign Intelligence Surveillance Act) process, which some in the party believe improperly sparked inquiries into Trump campaign dealings with Russia.

Barr was generally tight-lipped but said an inspector general of the Justice Department is preparing a report on the FISA process, which he said was expected to land in May or June.

Democrat Matt Cartwright of Pennsylvania, meanwhile, took Barr to task for the Justice Department declining to defend the Affordable Care Act. In a recent filing with a federal Appeals Court, the administration said the entire law, known as Obamacare, should be struck down as unconstitutional.

"I'm dismayed to hear that you're willing to drive our healthcare system off a cliff," said Cartwright.

Barr objected to the characterization, stressing the Justice Department doesn't set health care policy, and expressed confidence that the courts would soberly determine the issue.

"If you think it's such an outrageous position then you have nothing to worry about," said Barr. "Let the courts do their job."

Barr is slated to testify before a U.S. Senate appropriations panel on Wednesday.