Report doesn't exonerate Trump: Mueller

He reiterated he had not cleared President Donald Trump of obstruction of justice or, as the president has said, totally exonerated him.

Mueller appeared on Wednesday for eagerly anticipated testimony at the first of two back-to-back congressional hearings that carry high stakes for Trump and Democrats who are split between impeaching him or moving on to the 2020 election.

The former FBI director, who spent 22 months investigating Russian interference in the 2016 US election and Trump's conduct, appeared first before the House of Representatives Judiciary Committee.

The committee's Democratic chairman, Jerrold Nadler, praised Mueller and said no one, including Trump, is "above the law".

Mueller, 74, was surrounded by news photographers as he took his place in the packed hearing room, showing little apparent emotion as he scanned the scene.

"Obstruction of justice strikes at the core of the government's efforts to find the truth and to hold wrongdoers accountable," Mueller testified.

Trump has claimed that the Mueller inquiry resulted in the president's "complete and total exoneration". Asked by Nadler if he had exonerated Trump, Mueller said, "No."

Mueller, accused by Trump of heading a "witch hunt" and trying to orchestrate a "coup" against the Republican president, said his inquiry was conducted in "a fair and independent manner" and that members of the special counsel's team "were of the highest integrity".

"Let me say one more thing," Mueller said. "Over the course of my career, I have seen a number of challenges to our democracy. The Russian government's effort to interfere with our election is among the most serious."

In a comment sure to disappoint Republicans, Mueller said he would not answer questions about the origins of the Russia probe in the FBI before he was named to take over the inquiry in 2017 or about a controversial dossier compiled by a former British intelligence agent

Mueller was set to testify later in the day before the House Intelligence Committee. Democrats control the House, while Trump's fellow Republicans control the Senate.

The hearing provided Democrats a chance to air publicly and in plain language the key findings of the sometimes dense Mueller report.

Democrats entered the hearings hoping Mueller's testimony would rally public support behind their own ongoing investigations of the president and his administration. Democrats are deeply divided over whether to launch the impeachment process set out in the US Constitution for removing a president from office for "high crimes and misdemeanours".

Mueller's inquiry detailed numerous contacts between Trump's 2016 presidential campaign and Russia at a time when the Kremlin was interfering in the 2016 US election with a scheme of hacking and propaganda to sow discord among Americans and boost Trump's candidacy.

Mueller's investigative report said the inquiry found insufficient evidence to establish that Trump and his campaign engaged in a criminal conspiracy with Russia.

The report did not reach a conclusion on whether Trump committed the crime of obstruction of justice in a series of actions aimed at impeding the inquiry, but pointedly did not exonerate him. Attorney General William Barr, a Trump appointee, subsequently cleared the president of obstruction of justice.

In his opening statement, Mueller reiterated that his team had decided not to make a determination on the question of obstruction.

"Based on Justice Department policy and principles of fairness, we decided we would not make a determination as to whether the president committed a crime. That was our decision then and remains our decision today," Mueller said.

The committee's top Republican, Doug Collins, said the facts of the Mueller report are that "Russia meddled in the 2016 election. The president did not conspire with Russians. Nothing we hear today will change those facts.

"The president watched the public narrative surrounding the investigation assume his guilt while he knew the extent of his innocence," Collins said. "The president's attitude towards the investigation was understandably negative, yet the president did not use his authority to close the investigation."

Australian Associated Press