Bill Taylor, the top US diplomat in Ukraine, told impeachment hearings he had a “clear understanding” aid would be withheld pending an investigation.
Mr Trump denies there was any threat.
Mr Taylor testified in closed door impeachment hearings last month, and a transcript of his evidence was released to the public on Wednesday.
He told a Congressional committee that two state department officials said Mr Trump would not “sign a check” for nearly $400m (£311m) in US military aid until Ukrainian officials announced an investigation into Democratic presidential contender and former Vice-President Joe Biden.
Mr Biden’s son Hunter Biden previously sat on the board of the Ukrainian oil company Burisma. President Trump has claimed without evidence that Joe Biden used his position as vice-president to quash an investigation into Burisma.
Mr Taylor also told the Congressional committee that Mr Trump’s personal lawyer, Rudy Giuliani, was behind the drive to get Ukraine to announce an investigation into the Bidens.
Mr Taylor’s testimony followed evidence from Gordon Sondland, an ally of President Trump who is the US ambassador to the European Union, who told the impeachment hearings that he informed a Ukrainian official of a quid pro quo – US military aid in exchange for an investigation announcement. Mr Sondland testified that a White House visit for the Ukrainian president was also part of the deal.
Mr Sondland had returned to Congress to revise earlier testimony, in which he denied that any preconditions had been placed on the delivery of aid.
Also on Thursday, the Washington Post and the New York Times reported that President Trump asked that the attorney general, William Barr, hold a news conference to declare that the president had broken no laws during his call with the Ukrainian president – the very call that led to a whistleblower complaint against Mr Trump.
Mr Barr, a Trump appointee who has proved to be a staunch ally of the president, reportedly refused to do so.
Impeachment goes public
House Intelligence Committee chairman Adam Schiff, who is overseeing the inquiry, told reporters on Wednesday that the evidence for presidential wrongdoing was growing.
He said: “We are getting an increasing appreciation for just what took place during the course of the last year – and the degree to which the president enlisted whole departments of government in the illicit aim to get Ukraine to dig up dirt on a political opponent.”
The Capitol Hill hearings will be broadcast live, with both Democratic and Republican lawmakers questioning witnesses.
On Wednesday – a week ahead of his scheduled public hearing – House Democrats released a transcript of Mr Taylor’s evidence. It showed the diplomat told lawmakers it was his “clear understanding” that the president had withheld US military aid because he wanted Ukraine to investigate the Bidens.
The military aid to Ukraine was eventually released in September, after a whistleblower raised the alarm about a 25 July phone call in which Mr Trump asked the Ukrainian president to investigate the Bidens. The Department of Justice, led by the attorney general, Mr Barr, initially attempted to prevent the whistleblower complaint being passed to congress, but it was subpoenaed by House Democrats.