U.S. President Donald Trump has said the sources who gave information to the whistleblower whose explosive complaint was made public on Thursday were "close to a spy" and that spies were treated differently "in the old days."

The whistleblower has alleged that Trump pressured Ukrainian President Volodymyr Zelensky to dig for dirt about Joe Biden, a political opponent, using 400 million U.S. dollars in foreign aid money as leverage and that the White House tried to "lock down" evidence about Trump's conduct. 

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What are the key allegations and questions?

The scandal triggered moderate Democrats to move in favor of impeachment, and House Speaker Nancy Pelosi announced the launch of an inquiry into Trump on Tuesday.

'Spies and treason'

The complaint from the whistleblower, who the New York Times reported was a CIA officer, chimes with a partial transcript of a call released by the White House on Wednesday.

However, the publically released complaint reveals the whistleblower received information from several U.S. officials and "was not a direct witness to most of the events described."

In an audio recording obtained by the LA Times, Trump told American diplomats at a private event in New York: "Who's the person that gave the whistleblower the information? Because that's close to spy. 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."

James Clapper, the former director of national intelligence, told CNN the comments could amount to witness intimidation – treason is punishable by death. The Democratic chairmen of the House foreign affairs, intelligence, and oversight committees described the president's remarks as "reprehensible witness intimidation."

Who is the whistleblower?

The New York Times reported on Thursday that the whistleblower was "a CIA officer who once worked at the White House." Trump has previously suggested the whistleblower was a "partisan hack," but is not believed to know the person's identity.

Joseph Maguire, acting director of national intelligence, insisted during questioning in Congress on Thursday that he did not know the whistleblower's identity but would protect the person's anonymity. He added that the whistleblower had acted in good faith and followed the law in bringing the complaint.

However Maguire refused to say whether he had discussed the whistleblower's complaint with Trump, and insisted it would not be appropriate for him to respond to the president's criticisms of the whistleblower.

Democrats are working to clear legal hurdles to allow the whistleblower to testify before Congress, with CNN reporting that a tentative agreement is in place.

Trump lashes out

Trump denies any impropriety and has accused the Democrats of a "witch hunt."

On Thursday evening the president hit out at the impeachment inquiry, telling reporters "there should be a way of stopping it, maybe legally through the courts." 

Meanwhile Rudy Giuliani, the personal lawyer to Trump who has a central role in the scandal, told the Atlantic on Thursday that he was a "hero." 

"It is impossible that the whistleblower is a hero and I'm not," Giuliani said. "And I will be the hero! These morons – when this is over, I will be the hero."