US President Donald Trump has likened whistleblower sources to spies, before saying that "in the old days" things were handled "a little differently", according to audio obtained by US media outlets.

Key points: Mr Trump had earlier labelled the complaint as "fake news"

Mr Trump had earlier labelled the complaint as "fake news" The complaint was based on multiple accounts from the whistleblower's colleagues

The complaint was based on multiple accounts from the whistleblower's colleagues Acting director of National Intelligence Joseph Maguire told Congress the whistleblower "did the right thing"

Mr Trump was speaking at an event in New York, with staff from the US mission to the United Nations, when he said he wanted to know "who gave the whistleblower the information" that was used as the basis of an official complaint that sparked an impeachment inquiry into him.

The Los Angeles Times said it obtained audio of the comments from a person at the event and the New York Times reported the remarks "stunned people in the audience".

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 31 seconds 31 s Audio obtained by the LA Times of Donald Trump's comments about whistleblower sources.

"I want to know who's the person who gave the whistleblower the information because that's close to a spy," Mr Trump says in the audio clip.

"You know what we used to do in the old days when we were smart? Right? The spies and treason, we used to handle it a little differently than we do now."

Earlier in the day, US officials released a declassified version of the whistleblower complaint filed against Mr Trump, which alleged he used his office to solicit interference in the 2020 presidential election from a foreign country.

The whistleblower is still anonymous

The complaint explains the whistleblower's allegations are based on "more than half a dozen US officials" over the course of four months.

It alleges Mr Trump pressured Ukraine to investigate one of the US President's main political rivals, Joe Biden, during a July phone call with the country's President.

The whistleblower also alleged the White House moved to "lock down" records of the phone conversation in the days following the call, placing them in a more secure "codeword-level system" used to protect information sensitive to national security.

"This set of actions underscored to me that White House officials understood the gravity of what had transpired in the call," the whistleblower said.

Whistleblower 'did the right thing'

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 45 seconds 45 s US Intelligence Director supports whistleblower

Moments after the whistleblower report was made public, the committee began a hearing where acting director of National Intelligence Joseph Maguire was giving evidence about the document, which he had for weeks refused to share with Congress.

The unidentified whistleblower submitted the complaint to Michael Atkinson, the US government's intelligence inspector general, in August.

Mr Maguire then blocked release of the complaint to Congress, citing issues of presidential privilege and saying the complaint did not deal with an "urgent concern" — Mr Atkinson disagreed, but said his hands were tied.

Mr Maguire acknowledged that the complaint alleged serious wrongdoing by the President, but insisted that it was not his role to judge whether the allegations were credible or not.

He said he was unfamiliar with any other whistleblower complaint in American history that "touched on such complicated and sensitive issues."

"I believe that everything in this matter here is totally unprecedented," he said.

Mr Maguire told the committee the whistleblower "did the right thing" by coming forward to report his concerns, and followed the law "every step of the way".

In a statement, the Democratic chairman of the committee Adam Schiff said it was a "travesty" the complaint had not been released to politicians earlier.

Mr Schiff said the whistleblower had already faced "reprisals" from the Trump administration. ( AP: J Scott Applewhite )

"This complaint should never have been withheld from Congress. It exposed serious wrongdoing, and was found both urgent and credible by the inspector general," he said.

"This complaint is a roadmap for our investigation, and provides significant information for the committee to follow up on with other witnesses and documents."

Mr Schiff thanked the whistleblower for coming forward, and said that individual had already faced "reprisals" from the Trump administration.

Joe Biden brushes off controversy on campaign trail

"This isn't about me," Mr Biden told a few dozen guests gathered by a backyard swimming pool at a fundraiser in San Marino, California.

"It's a tactic that's used by this President to try to hijack an election so we do not focus on the issues that matter in our lives."

The former vice president remains the frontrunner in the 2020 Democratic presidential race, despite a surge by Senator Elizabeth Warren in recent weeks.

Mr Trump continued his attacks on Mr Biden and Democrats late Thursday (local time), tweeting:

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