US president Donald Trump urged the new leader of Ukraine to investigate the son of former vice president Joe Biden this summer, according to reports.
Democrats have condemned what they see as a clear effort to damage a political rival widely seen as the man who will challenge Trump for the presidency next year.
The allegations are at the centre of an explosive whistleblower complaint against Trump.
The escalating controversy has created a showdown between congressional Democrats and the Trump administration, which has refused to turn over the formal complaint by a national security official or even describe its contents.
Trump is defending himself against the intelligence official’s complaint, asserting that it comes from a “partisan whistleblower”, though the president also said he does not know who might be behind it.
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