Donald Trump has said he wants Australia’s role in setting off the FBI inquiry into links between Russia and his election campaign examined by the US attorney general, William Barr.

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

A spokesman for Marise Payne, Australia’s foreign affairs minister, responded to the comments on Saturday by saying: “Australia and the United States are the closest of allies.”

“The government has not commented on these matters and doing so could prejudice any ongoing investigation,” he said.

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

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

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

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

In an interview with the ABC’s Matt Bevan, Downer said Papadopoulos told him during the meeting he was confident Trump would win the election because the Russians had some information on Clinton which could be damaging to her if released. Downer then passed this information on to Canberra.

Papadopoulos denies he ever mentioned Russia and Clinton during the meeting.

After being forwarded to Canberra the information was 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, said the Papadopoulos-Downer meeting was what prompted the FBI to open its inquiry on 31 July 2016.

“This sort of idea that there is a kind of a Asis-Asio-MI6-MI5-FBI-CIA-Ukrainian government conspiracy to bring down the Trump administration, that this is treason, that I should be in Guantanamo Bay... I mean it’s a little bit sad that people take that kind of thought seriously,” Downer told the ABC.

He added that he was just doing his job as a diplomat at the time.

“If people tell me things like that, which I think are damaging to western security interests ... expect me, Alexander Downer, former for minister of Australia, to be supporting western interests against Russian intelligence. You can count on me.”

The FBI investigation led to Mueller being appointed.

On Friday, Trump described the Russia inquiry as “an attempted coup or an attempted takedown of the president of the United States”.

He also said he might ask the 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,” 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,” he said.

Papadopoulos was one of Mueller’s first convictions – the former aide had pleaded guilty to lying to the FBI. He was sentenced to 14 days’ jail.

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.”