Former special counsel Robert Mueller admitted under questioning that President Donald Trump tried to "exert undue influence" over his Russia investigation and was not exonerated by the final report in more than six hours spent testifying on Wednesday.

Mueller made those comments in the first of two House committee hearings, dealing with potential obstruction of justice. The afternoon's intelligence committee session began to dive into alleged ties between the Trump campaign and the Kremlin.

House intelligence chair Adam Schiff, Democrat from California, framed Mueller's report as telling the story of "disloyalty to country, about greed and about lies."

Republican counterpart Devin Nunes portrayed the day's hearings in contrast as the "last gasp of the Russia collusion conspiracy."

Schiff was able to get Mueller to answer one of the president's favourite characterizations of his probe.

"It is not a witch hunt," Mueller responded.

As well, Mueller agreed with Schiff's assessment that elected officials should be held "to a standard higher than mere avoidance of criminality."

Intelligence committee members Adam Schiff, left, and Devin Nunes, right, in their opening statements offered wildly different portrayals of the special counsel investigation into links between Donald Trump's campaign and Russia. (The Associated Press)

The morning session, before the House judiciary committee, focused on whether Trump illegally obstructed justice by attempting to seize control of Mueller's investigation into whether Russia interfered in the 2016 U.S. election.

Questioned by judiciary chair Jerry Nadler, a Democrat from New York, Mueller agreed the report did not exonerate Trump of obstruction of justice.

"The president was not exculpated for the acts that he allegedly committed," Mueller said in his first Capitol Hill appearance in Washington since wrapping his two-year Russia probe in late March.

Mueller agreed Trump refused to sit down for questioning with the special counsel, despite attempts lasting nearly a year to negotiate an interview.

Mueller testified in the afternoon session that he chose not subpoena Trump, because it was expected it would be challenged by the president's legal team, potentially tying up the probe for months in the courts. Trump's written responses "were not as useful" as an in-person would have been, he added.

Praise of WikiLeaks 'problematic'

Mueller testified he believed it was possible a president could be indicted after leaving office, but that his investigation was guided by Department of Justice guidelines that advise that a sitting president can't be indicted.

The most bracing Mueller got in his personal criticism of the president was to refer to Trump's frequent expressions of approval for WikiLeaks releasing information damaging to the Democrats during the 2016 presidential campaign.

"Problematic is an understatement," he said.

Mueller speaks to the willingness of a campaign to receive information from foreign sources damaging to their opponent. 0:36

However, Trump described the day's hearings as "all nonsense," dismissing the Russia probe and saying it created a "phony cloud" over his administration.

The president said Mueller's performance was "obviously not very good" and accused him of not knowing about certain details of his investigation. He went on to call it a "devastating day" for the Democrats.

Trump spoke to reporters at the White House before leaving for Wheeling, W. Va., where he's attending a private fundraiser for his re-election campaign.

The Trump campaign sent out a personal plea to donors Wednesday to tell Democrats to end the "WITCH HUNT" by raising $2 million US in 24 hours.

Trump speaks to the media about Robert Mueller's comments before departing the White House for West Virginia. (Mary F. Calvert/Reuters)

Georgia Republican Doug Collins said the investigation has been completed and is now just a distraction from the important work Congress needs to do.

"No American conspired to throw the election."

Collins said "baseless gossip" helped launch the investigations into the president. The Republicans have focused on the intelligence document compiled by Christopher Steele that contained information since proved to be untrue. Democrats have made the counterargument the dossier was but one thread of a sprawling investigation.

Attorney General William Barr has signed off on an investigation into the origins of the FBI probe into the president, which predated Mueller's appointment.

Texas Republican Louie Gohmert assailed Mueller for the presence of Peter Strzok on his team. Strzok was an FBI investigator who worked on behalf of the special counsel team for the first few months of its existence, and was reassigned soon after it was discovered he had sent derogatory texts about Trump during the 2016 campaign.

Rep. Louie Gohmert, a Texas Republican, questions Mueller. (Win McNamee/Getty Images)

"Peter Strzok hated Trump. You didn't know that before he was made part of your team?" asked Gohmert.

Meanwhile, fellow Texas Republican John Ratcliffe accused Mueller of breaching Justice Department guidelines by offering opinions on Trump's behaviours.

The special counsel detailed several instances in which Trump tried to interfere with the investigation or derail it, and occasions where the president dangled the possibility of pardons to associates who were being questioned by authorities.

"If we had confidence after a thorough investigation of the facts that the president clearly did not commit obstruction of justice, we would so state," the report stated.

In his report, Mueller left open the possibility that Trump engaged in his behaviours not necessarily due to criminal reasons but for personal and political calculations. The report also stated that Trump's efforts to influence the investigation were unsuccessful mainly because persons around him declined to carry out his orders.

The panel heard repeated references to Trump trying, through White House counsel Don McGahn, to have Mueller removed as special counsel.

Mueller agreed with Rhode Island Democrat David Cicilline that "an unsuccessful attempt to obstruct [justice] is still a crime."

Reluctant witness, sometimes stilted

Mueller, 74, had given indications ahead of Wednesday that he would not stray beyond his report.

He frequently gave terse answers to questions and referred back to the wording in his report. At times, he appeared stilted and halting, and said he couldn't answer certain questions, earning the disapproval of Republicans such as Matt Gaetz of Florida.

Republican congressman Matt Gaetz becomes visibly frustrated as former U.S. special counsel Robert Mueller refuses to answer repeated questions on the Christopher Steele dossier during his testimony. 2:30

In the afternoon, Mueller had to walk back a statement he made to Democrat Ted Lieu in the first session in which he appeared to indicate the Justice Department guidelines on indicting a sitting president were what prevented the special counsel from making a final judgment on the president's culpability.

I've been in this business for almost 25 years. In those 25 years I've not had occasion once to ask somebody about their political affiliation. It is not done. - Robert Mueller

"We did not reach a determination as to whether the president committed a crime," he said.

At other points, it appeared evident Mueller delegated much work to his team. He told Republican Martha Roby of Alabama he attended "very few" of approximately 500 interviews conducted.

Mueller did become animated when North Dakota Republican Kelly Armstrong pointed to donations to Democratic candidates made by some investigators on his team.

"I've been in this business for almost 25 years. In those 25 years I've not had occasion once to ask somebody about their political affiliation," said Mueller. "It is not done. What I care about is the capability of the individual to do the job."

He also pointed out that most of the 19 lawyers had been assigned to him by the Department of Justice.

Trump, Mueller clash on May 2017 meeting

Mueller was appointed special counsel on May 17, 2017, and tasked with looking into "any links and/or co-ordination between the Russian government and individuals associated with the campaign of President Donald Trump." His appointment came days after Trump fired FBI director James Comey.

"Our investigation found that the Russian government interfered in our election in sweeping and systematic fashion," Mueller said in his opening statement.

Trump has said the redacted report cleared him of allegations of collusion of Russia. But the report took pains to explain that collusion is not a specific offence in federal law, though conspiracy is.

Mueller looks at documents as he testifies before Congress on Wednesday. (Saul Loeb/AFP/Getty Images)

Mueller rebutted Trump's claim that he had conflicts of interest. Specifically, Mueller said he was not a candidate to return as FBI director when he had a meeting with Trump after Comey was let go and shortly before being appointed as special counsel.

He characterized it is an informational interview.

"I was asked to give my input on what it would take to do my job," said Mueller, who previously served as agency head between 2001 and 2013.

Trump tweeted Wednesday that Vice-President Mike Pence and other witnesses would agree Mueller applied and interviewed for the job and was "turned down" for it.

'I hope this is not the new normal'

Several Trump associates were found to have lied to federal investigators in the probe, and his former campaign manager and former personal attorney are serving prison sentences for various offences.

Robert Mueller delivers his opening statement to judiciary and intelligence committees 5:14

With respect to a controversial Trump Tower meeting in 2016 with Russians who promised dirt on Hillary Clinton, the special counsel concluded it wasn't clear the Trump campaign members in attendance had wilful intent or knew they might be breaking the law.Mueller would not answer as to whether he wished to interview Donald Trump Jr., who OK'd that meeting.

When asked by Vermont Democrat Peter Welch to address the fact overtures from foreign entities weren't reported to authorities, Mueller said, "I hope this is not the new normal, but I fear it is."

Two things Mueller has made clear this morning:<br>✔️ He did not find “no collusion” with Russia<br>✔️ He did not “totally exonerate” the President<br>No matter how many times Trump repeats those lies it doesn’t make them true.<a href="https://twitter.com/hashtag/MuellerReport?src=hash&ref_src=twsrc%5Etfw">#MuellerReport</a> <a href="https://twitter.com/hashtag/MuellerHearings?src=hash&ref_src=twsrc%5Etfw">#MuellerHearings</a> —@RepBarbaraLee

Ohio Republican Steve Chabot derided the opposition's motives for bringing Mueller to testify, accusing the Democrats of trying to build up "a groundswell of support" for impeachment.

Currently, about 90 of the 235 Democrats in the House — where an impeachment inquiry would begin — support launching such hearings, as does former Republican Justin Amash, now an Independent. But Democratic House Speaker Nancy Pelosi has been cool to the prospect of impeachment, preferring that the American people to deliver a verdict on Trump in the 2020 election.

Impeachment is the political process of removing from office certain elected or public officials accused of wrongdoing. The process is more difficult than you might think. 2:01

Mueller told Louisiana Republican Mike Johnson he would not speculate about impeachment.

Based on tweets from sitting members on Wednesday, it was unlikely that a bipartisan consensus of opinion would emerge on impeachment hearings.