The complaint, which the administration has refused to let congress see, remains shrouded in mystery but is "serious" and "urgent", the government's intelligence watchdog says.

But Trump has dismissed the matter, insisting he did nothing wrong.

He said the complaint was made by a "partisan whistleblower" although he later said he did not know the identity of the person.

He chided reporters for asking about it and said the complaint was "just another political hack job".

"I have conversations with many leaders. It's always appropriate. Always appropriate," Trump said. "At the highest level always appropriate. And anything I do, I fight for this country."

Some of the whistleblower's allegations appear to centre on Ukraine, according to the Washington Post and the New York Times.

Trump was asked at the Oval Office if he knew if the whistleblower's complaint centred on a July 25 phone call with Ukrainian President Volodymyr Zelensky.

The US president responded: "I really don't know" but continued to insist any phone call he made with a head of state was "perfectly fine and respectful".

He berated reporters for asking about the whistleblower complaint.

The stand-off raises fresh questions about the extent to which Trump's allies are protecting the Republican president from oversight and, specifically, whether his new acting director of national intelligence, Joseph Maguire, is working with the Justice Department to shield the president from the reach of congress.

It also plunged the Trump administration into an extraordinary showdown with congress over access to the whistleblower's complaint as legislators press their oversight of the executive branch.

The administration is keeping congress from even learning what the whistleblower is alleging, but the intelligence community's inspector general said the matter involves the "most significant" responsibilities of intelligence leadership. A legislator said the complaint was "based on a series of events".

The inspector general appeared before the House intelligence committee behind closed doors on Thursday but declined, under administration orders, to reveal the substance of the complaint.

Adam Schiff, the Democratic chairman of the committee, said he was prepared to go to court to try to force the Trump administration to open up about the complaint.

"The inspector general has said this cannot wait," said Schiff, describing the administration's blockade as an unprecedented departure from law.

"There's an urgency here that I think the courts will recognise."

Schiff said he could not confirm whether newspaper reports were accurate because the administration was claiming executive privilege in withholding the complaint, but letters from the inspector general to the committee released on Thursday said it was an "urgent" matter of "serious or flagrant abuse" that must be shared with legislators.

The letters also made it clear that Maguire consulted the Justice Department in deciding not to transmit the complaint to congress in a further departure from standard procedure. It is unclear whether the White House was also involved, Schiff said.

Trump on Thursday dismissed it all, noting that others would be aware of the call.

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

House Democrats are fighting the administration separately for access to witnesses and documents in impeachment probes.

Democrats are also looking into whether Trump lawyer Rudy Giuliani travelled to Ukraine to pressure the government to aid the president's re-election effort by investigating the activities of potential rival Joe Biden's son Hunter, who worked for a Ukrainian gas company.

During an interview on CNN, Giuliani was asked whether he had asked Ukraine to look into Biden. Giuliani initially said: "No, actually I didn't," but seconds later he said: "Of course I did."

Australian Associated Press