US President Donald Trump wants Australia's role in sparking the 2016 FBI probe into potential links between his election campaign and Russia examined by US Attorney General William Barr.

In a potentially explosive development for the historically rock solid US-Australian alliance, Mr Trump has publicly named Australia for the first time while discussing what he calls the "Russia hoax" and "witch hunt".

The move was denounced by some members of US Congress who predicted trust between the Five Eyes intelligence sharing nations - the US, Australia, UK, Canada and New Zealand - could be eroded.

Mr Trump said he has declassified potentially millions of pages of intelligence documents related to surveillance activities on his campaign and Mr Barr would have "full and complete authority" to examine them.

"So what I've done is I've declassified everything," Trump told reporters at the White House on Friday before departing on a trip to Japan.

"He can look and I hope he looks at the UK and I hope he looks at Australia and I hope he looks at Ukraine.

"I hope he looks at everything, because there was a hoax that was perpetrated on our country."

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US Special Counsel Bob Mueller's report on links between the Trump campaign and Russia, pointed to a 2016 meeting between then Trump campaign aide George Papadopoulos and Australian high commissioner to the UK Alexander Downer in a London bar as prompting the FBI to open its Trump-Russia probe.

AAP

The FBI probe led to Mr Mueller being appointed as special counsel.

Mr Papadopoulos has claimed Mr Downer spied on him during the bar meeting, a claim which Mr Downer has rejected.

Mr Downer did say Mr Papadopoulos told him at the bar Russia had damaging material on Trump's presidential rival Hillary Clinton.

The information was forwarded to Canberra and then passed on to US intelligence services and the FBI.

Mr Papadopoulos denies telling Downer anything about Russia obtaining damaging information on Ms Clinton.

Mr Trump on Friday described the Russia probe as "an attempted coup or an attempted takedown of the president of the United States".

Mr Trump also said he might ask outgoing British Prime Minister Theresa May about "potential Five Eyes spying" on his campaign.

"I may very well talk to her about that, yeah," Mr Trump said.

"There's word and rumour that the FBI and others were involved, CIA were involved, with the UK, having to do with the Russian hoax," Trump said.

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Jim Himes, a Democrat member of the House Intelligence Committee, said Trump was damaging alliances and potentially exposing confidential sources for his own political purposes.

"What the UK and Australia and New Zealand see is because the president, in order to forward a political fantasy, may blow our sources and methods, put our people at risk," Mr Himes told CNN.

"This is a very dangerous thing for the United States.

.....The Dems want a second shot at Bob Mueller, are very unhappy with the No Collusion Report. They should not be allowed to play this game any longer - no second chances - must get back to work. So bad for our Country! — Donald J. Trump (@realDonaldTrump) May 24, 2019

Mr Papadopoulos was one of Mueller's first convictions, with the former aide pleading guilty to lying to the FBI. He was sentenced to 14 days' jail.

Mr Papadopoulos, Republican members of Congress and right-wing US media figures have been urging the president to declassify the documents.

"It's the greatest hoax, probably, in the history of our country and somebody has to get to the bottom of it," Trump said.

"We'll see.

"But for a long period of time, they've wanted me to declassify and I did."

Criticism builds

Mr Trump is defending his unprecedented decision to give his Justice Department chief unfettered access to the country's deepest foreign intelligence secrets amid an outcry from intelligence officials.

The president said Mr Barr needed unilateral power to declassify any top secret material to get to the roots of the 2016-2018 investigation into whether his election campaign colluded with Russia.

Barr "will be able to see how this hoax, how the hoax or witch hunt started, and why it started," Trump said.

"It was an attempted coup or an attempted takedown of the president of the United States. It should never ever happen to anybody else, so it's very important."

But politicians and former intelligence community leaders said Mr Trump and Mr Barr are threatening to expose the country's most protected sources of secrets on Russia to mount a political attack on a legitimate investigation that exposed a serious threat to the United States.

Former CIA deputy director John McLaughlin called for Congress to thwart the move.

"Giving Barr declassification authority for this investigation is a really bad idea," he said on Twitter.

"The agencies can cooperate but must retain their legal responsibility for protecting sources."