Members of US Congress have seen for the first time a whistleblower complaint against Donald Trump that has led to calls for his impeachment.
One Democrat described it as “disturbing” while a Republican said it was “troubling”.
The complaint refers to a controversial phone call between Mr Trump and the Ukrainian president, US media say.
The latest developments come as the acting director of US intelligence is due to testify on the issue.
Joseph Maguire’s account of the complaint by an unnamed intelligence officer will be closely scrutinised when he appears before members of the House Intelligence Committee later on Thursday.
The content of the complaint is still classified but the most senior Democrat in the US Senate, Chuck Schumer, has called for its immediate release.
During a news conference on Wednesday evening, President Trump again dismissed the impeachment proceedings as a “hoax” and a “witch-hunt”.
According to notes of the 25 July phone call, President Trump asked his Ukrainian counterpart, Volodymyr Zelensky, to look into corruption claims involving the son of Joe Biden, Mr Trump’s possible rival in next year’s presidential election. There is no evidence of any wrongdoing by the Bidens.
In a tweet on Wednesday, Mr Biden accused President Trump of “an abuse of power”.
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Donald Trump pressured Ukraine to manufacture a smear against a domestic political opponent — the “transcript” made that clear. It’s an abuse of power that violates the oath of office and undermines our democracy.
Congress must hold him accountable.
— Joe Biden (@JoeBiden) September 26, 2019