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

It is a potentially explosive development for the historically solid US-Australian alliance and the first time Mr Trump has publicly named Australia while discussing what he calls the “Russia hoax” and “witch hunt”.

Mr Trump said on Saturday (AEST) he has declassified “potentially millions of pages” of intelligence documents related to surveillance activities on his campaign and Mr Barr would be in charge of analysing it.

“So what I’ve done is I’ve declassified everything,” Mr Trump told reporters at the White House 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.”

President Donald Trump and first lady Melania leave the White House for Japan. Photo: AAP

Mr Trump’s former campaign aide George Papadopoulos has claimed Australia’s former high commissioner to the UK, Alexander Downer, allegedly spied on him during a meeting at a London bar in May, 2016.

Mr Downer has rejected this, but said that during the meeting Mr Papadopoulos told him Russia had damaging material on Mr Trump’s presidential rival Hillary Clinton.

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

US Special Counsel Bob Mueller, in his report on the links between the Trump campaign and Russia, stated the Papadopoulos-Downer meeting was what prompted the FBI to open its probe on July 31, 2016.

The FBI investigation led to Mr Mueller being appointed.

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.

Five Eyes is the intelligence sharing alliance between the US, Australia, UK, Canada and New Zealand.

“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,” Mr Trump said.

Mr Papadopoulos was one of Mr 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,” Mr Trump said.

“We’ll see.

“But for a long period of time, they’ve wanted me to declassify and I did.”

-AAP