US President Donald Trump  Photo:Xinhua

A whistleblower report that has prompted an impeachment inquiry into Donald Trump contains "very troubling" allegations, US lawmakers have said, after the president dismissed the case against him as a "joke."

Democrats have accused Trump of a "mafia-style shakedown" of his Ukrainian counterpart Volodymyr Zelensky by urging a probe into his rival Joe Biden - prompting the complaint by an anonymous US intelligence official earlier in September.

That document has now been declassified, Republican Congressman and House Intelligence Committee member Chris Stewart said late on Wednesday, and CNN reported that it could be released as soon as Thursday morning.

Top Democratic Senator Chuck Schumer told reporters the complaint was "very troubling" - an assessment echoed by Republican Senator Ben Sasse, who said that "there's obviously lots that's very troubling there."

Trump struck an uncharacteristically subdued tone on Wednesday at his first news conference since Democrats launched an official impeachment inquiry.

"They are getting hit hard on this witch hunt because when they look at the information, it's a joke," said the president.

A more defiant Trump had earlier insisted he exerted "no pressure" on Kiev - a claim echoed by Zelensky, who appeared side-by-side with the US leader at a long-planned meeting on the sidelines of the UN General Assembly.

But the publication of a call transcript released by the White House on Wednesday sent new shockwaves through Washington, including within the ranks of Trump's Republican Party.

Trump and his allies claimed the July 25 call contained no evidence of a quid pro quo pressuring Zelensky to probe the president's top Democratic rival for the White House.