The US Justice Department said on Sunday that special counsel Robert Mueller's probe did not find evidence that President Donald Trump's campaign 'conspired or coordinated' with Russia to influence the 2016 presidential election.

Mueller also investigated whether Trump obstructed justice but did not come to a definitive answer.

In a four-page letter to Congress, Attorney-General William Barr quoted Mueller's report as stating it 'does not exonerate' the president on obstruction. Instead, Barr said, it 'sets out evidence on both sides of the question'.

Trump, in Florida, celebrated that the report showed 'there was no collusion.' He also claimed it showed there was no obstruction.

Barr released his summary of Mueller's report Sunday afternoon. Mueller wrapped up his investigation on Friday with no new indictments, bringing to a close a probe that has shadowed Trump for nearly two years. But the broader fight is not over.

The justice department summary sets up a battle between Barr and Democrats, who called for Mueller's full report to be released and vowed to press on with their own investigation.

For Trump, Barr's report was a victory on a key question that has hung over his presidency from the start: Did his campaign work with Russia to defeat Democrat Hillary Clinton?

Still, Mueller's investigation left open the question of whether Trump obstructed justice by firing FBI Director James Comey and drafting an incomplete explanation about his son's meeting with a Russian lawyer during the campaign. That left it to the attorney general to decide. After consulting with other department officials, Barr said he and his deputy, Rod Rosenstein, determined the evidence 'is not sufficient to establish that the president committed an obstruction of justice offence'.

Barr, nominated to his job by Trump last fall, said their decision was based on the evidence uncovered by Mueller and not based on whether a sitting president can be indicted.

A copy of a letter from Attorney-General William Barr advising Congress of the principal conclusions reached by Robert Mueller. (AP Photo)

Trump was at his Florida estate when lawmakers received the report. Barr's chief of staff called Emmet Flood, the lead White House lawyer on the investigation, to brief him on the findings shortly before he sent it to Congress.

Mueller's probe ensnared nearly three dozen people, senior Trump campaign operatives among them. The investigation illuminated Russia's assault on the American political system, painted the Trump campaign as eager to exploit the release of hacked Democratic emails to hurt Democrat Hillary Clinton and exposed lies by Trump aides aimed at covering up their Russia-related contacts.

Mueller submitted his report to Barr instead of directly to Congress and the public because, unlike independent counsels such as Ken Starr in the case of President Bill Clinton, his investigation operated under the close supervision of the US Justice Department, which appointed him.

The House Judiciary Committee chairman says Congress needs to hear from Barr about his decision and see 'all the underlying evidence'.

Mueller 'clearly and explicitly is not exonerating the President, said Rep. Jerry Nadler, D-N.Y. in a series of tweets, but Barr took two days 'to tell the American people that while the President is not exonerated, there will be no action by DOJ'.

'There must be full transparency in what Special Counsel Mueller uncovered to not exonerate the President from wrongdoing. DOJ owes the public more than just a brief synopsis and decision not to go any further in their work,' Nadler tweeted

Barr said that Mueller 'thoroughly' investigated the question of whether the Trump campaign coordinated with Russia's election interference, issuing more than 2,800 subpoenas, obtaining nearly 500 search warrants and interviewing 500 witnesses.

He said Mueller also catalogued the president's actions including 'many' that took place in 'public view,' a possible nod to Trump's public attacks on investigators and witnesses.

In the letter, Barr said he concluded that none of Trump's actions constituted a federal crime that prosecutors could prove in court.

Democrats are reminding that the House voted nearly unanimously, 420-0, to release the full Mueller report, which they say is more important not than ever. 'This is about transparency and truth — and a 4 page summary from Trump's AG doesn't cut it,' tweeted Rep. Mark Takano, D-Ca., the chairman of the Veterans Affairs Committee. 'The American people deserve to see the whole thing.'