A US intelligence official has filed a formal whistleblower report alleging US President Donald Trump made a troubling "promise" to an unidentified foreign leader, according to the Washington Post.

Key points: The whistleblower's complaint was lodged on August 12

The whistleblower's complaint was lodged on August 12 Mystery surrounds who the unidentified leader was or what the alleged promise may have involved

Mystery surrounds who the unidentified leader was or what the alleged promise may have involved Mr Trump brushed off the Washington Post's report and said he was not "dumb enough" to say anything inappropriate to a world leader

The Democratic chairman of the US House Intelligence Committee has since accused the Justice Department of blocking intelligence officials from providing Congress with details of the complaint.

Adam Schiff levelled the charge after a closed-door briefing with the inspector general for US spy agencies.

"I don't know whether the White House is directly involved, because we can't get an answer to that question, but we do know that they are making some claim that a privilege may apply," Mr Schiff told reporters.

Intelligence community inspector general Michael Atkinson told the committee in a September 9 letter that he had received a complaint he deemed urgent and credible.

But he said acting director of National Intelligence (DNI) Joseph Maguire found that it did not meet the definition of an "urgent concern" that would require its disclosure to Congress under an existing legal statute.

Mr Schiff says the national intelligence director was required to provide the complaint to Congress. ( AP: J Scott Applewhite )

In a subsequent letter on Tuesday, Mr Atkinson said Mr Maguire — after consulting with the Justice Department — had found the complaint was not subject to the statute because it did not concern conduct by an intelligence official or an intelligence activity overseen by the DNI.

Because of this finding, Mr Atkinson was unable to provide details on the complaint in speaking with the committee.

What did the whistleblower say?

The Washington Post published a report on Wednesday night local time, citing two former intelligence officials whose identities were kept anonymous.

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It was not made clear which leader the complaint referred to or what the President is alleged to have promised.

However, one of the former officials told the Washington Post the complaint, lodged with the inspector general of the intelligence community, was related to a phone call with Mr Trump.

The complaint was filed on August 12, when Mr Trump was on holiday at his New Jersey estate.

Mr Trump had interactions with at least five foreign leaders in the five weeks leading up to the complaint, the Washington Post reported.

Reuters and the Associate Press have not confirmed the report.

The White House declined to comment before the article was published.

A day later, Mr Trump dismissed the Washington Post report as "fake news" and "presidential harassment" on Twitter.

"Virtually anytime I speak on the phone to a foreign leader, I understand that there may be many people listening from various US agencies, not to mention those from the other country itself," he said.

"Is anybody dumb enough to believe that I would say something inappropriate with a foreign leader while on such a potentially 'heavily populated' call?"

Suspicions surrounding alleged pledge

Details of the whistleblower complaint are scant, however, some have suggested it could have related to a conversation between Mr Trump and Russian leader Valdimir Putin.

In an analysis piece on Thursday, Washington Post journalist James Hohmann said the alleged "promise" came after the White House scaled back its communications of talks between Mr Trump and other world leaders.

Mr Trump with Mr Putin at the Presidential Palace in Helsinki in July, 2018. ( AP: Jussi Nukari/Lehtikuva )

He pointed to a tweet from Russia detailing a July call between Mr Trump and Mr Putin, which was confirmed by the White House after the Russian statement was published.

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The statement said Mr Trump "offered assistance to Moscow in fighting wildfires in Siberia".

"The Russian president viewed Trump's offer as a sign that fully-fledged bilateral relations could be restored in the future."

However, others have pointed out that Mr Trump interacted with several world leaders in the weeks before August 12.

Those leaders include North Korea's Kim Jong-un, France's Emmanuel Macron, Egyptian President Abdel Fattah Al Sisi and President of Ukraine Volodymyr Zelenskyy.

He also spoke with Boris Johnson, who has since become the British Prime Minister, the American Broadcasting Company reported.

Protecting the President?

CNN said on Thursday that both the White House and Justice Department were involved in advising Mr Maguire not to give Congress a copy of the complaint.

The White House did not immediately respond to a request for comment, while a spokeswoman for the Justice Department referred to the two letters Atkinson had sent the committee. Mr Maguire's office did not respond to a request for comment.

Mike Quigley, a Democratic member of the committee, accused Attorney General William Barr of trying to protect Mr Trump.

"Mr Barr and the Department of Justice's job in their mind is to protect the President," he told reporters.

"And it doesn't matter if that violates the laws."

Mr Schiff issued a subpoena to Mr Maguire last week in an attempt to force him to turn over the whistleblower's complaint.

He said the complaint had not been transmitted to Congress within 10 days, which was a "violation of the law".

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On Thursday Mr Schiff said the committee could be forced to go to court to seek an order compelling officials to disclose the complaint.

"I hope that the director of national intelligence will reconsider because it's my understanding that by law he can provide this to us and by law he's required to provide this to us," he said.

Mr Maguire is scheduled to testify in public to the panel next week.

ABC/Wires