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.Two competing narratives immediately took hold.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.Democrats meanwhile held it up as a smoking gun."This is how a mafia boss talks," charged senior lawmaker Adam Schiff.The call summary - which is not a verbatim transcript - shows Trump saying US Attorney General Bill Barr and the president's personal lawyer Rudy Giuliani would be in touch about probing the Ukraine-related activities of Biden and his son.In announcing the impeachment probe, House Speaker Nancy Pelosi said Trump's actions amounted to a betrayal of his oath of office and of national security.The next explosive episode in the rapidly unfolding drama is set for Thursday, when acting director of national intelligence Joseph McGuire testifies on Capitol Hill.A 'favour'As Barack Obama's vice president, Biden and other Western leaders pressured Ukraine to get rid of the country's top prosecutor, Viktor Shokin, because he was seen as not tough enough on corruption."There's a lot of talk about Biden's son, that Biden stopped the prosecution and a lot of people want to find out about that so whatever you can do with the attorney general would be great," Trump told Zelensky in the call."Biden went around bragging that he stopped the prosecution so if you can look into it... It sounds horrible to me." The transcript did not show Trump explicitly tying aid to Ukraine to Zelensky probing Biden, which has fed the calls to impeach him.But the two did discuss a major aid package, which had been frozen by Trump at the time and was only released in recent weeks.The memo also shows Trump asking Zelensky for a "favour" on an issue unconnected to Biden - just after noting that the US had been "very, very good to Ukraine".Analysts noted that the transcript uses ellipses - denoting words or entire passages are missing - on three occasions, all when Trump is making requests of Zelensky.Others pointed out that it was supposed to summarise a 30-minute call, yet the transcript runs to around just 12 minutes.'Witch hunt'Faced with the possibility of becoming the third US president in history to be impeached, Trump has pushed back hard - branding the probe the "single greatest witch hunt in American history... a disgraceful thing".Pelosi had for months resisted pressure from the party rank-and-file for impeachment, preferring to focus on next year's election fight.She relented on Tuesday, 11 days after the existence of the whistleblower complaint was made public. Democrats made clear the White House's release of the call record would not satisfy their investigation into whether the US leader broke the law, and the complaint has now been reviewed by lawmakers.