Donald Trump was "inappropriate" and "improper" to ask his Ukrainian counterpart to investigate Joe Biden, a White House official has told the impeachment inquiry into the US president.
During the first televised hearing of the inquiry, army veteran Lieutenant Colonel Alexander Vindman said he "couldn't believe what I was hearing" when the call between Mr Trump and Volodymyr Zelenskiy took place on 25 July.
Mr Trump is accused of asking Mr Zelenskiy to carry out two investigations to help his re-election - one targeting his rival and former vice president Mr Biden, the other involving a debunked theory that Ukraine interfered in the 2016 presidential election rather than Russia.
Lt Col Vindman, who wore his military uniform and medals as he gave evidence, said: "It was inappropriate, it was improper for the president to request - to demand - an investigation into a political opponent.
He added: "Frankly, I couldn't believe what I was hearing."
Lt Col Vindman was forced to fend off Republican efforts to cast doubt on his competence and loyalty to the US, answering questions regarding his background as an immigrant from the Soviet Union.
His family fled four decades ago when he was three, and he batted away suggestions he had an offer to become a minister of defence in Ukraine as "comical".
As well as having doubts cast over his evidence by Republicans in the room, Lt Col Vindman faced attacks from the president as he spoke to reporters at the White House.
Mr Trump said the inquiry was "a disgrace" and "an embarrassment to our nation", adding: "I'm going to let people make their own determination. I never heard of any of these people."
Giving evidence alongside Lt Col Vindman was Jennifer Williams, an aide to vice president Mike Pence.
She had also raised concerns about the phone call between the US and Ukrainian presidents.
Ms Williams was asked when she first knew of the $400m in military assistance being withheld from Ukraine, a decision she said was made "at the direction of White House chief of staff".
She said she first heard about the strategy on 3 July, several weeks before the phone call.
Also giving evidence on Tuesday was Mr Trump's former Ukraine envoy Kurt Volker, who resigned after being named in the whistleblower complaint that sparked the inquiry.
He told the hearing he did not knowingly take part in an effort to press Ukraine to investigate Mr Biden.
But he admitted he should have realised that Mr Trump was holding up military aid to Ukraine as a way to pressure the country to investigate his political rivals.
And while he was called to testify by Republicans, he said that the criticism of Mr Biden was "not credible" and that he would not act in any way other than in the national interest.
Following the hearing, US Senate majority leader Mitch McConnell provided a worthwhile reminder that, despite the drama of television coverage, the impeachment inquiry is likely to be a futile endeavour for the Democrats.
While the party should have the numbers to get impeachment through the House of Representatives, which they control, they will not have enough to deal the decisive blow in the Republican-controlled Senate.
Mr McConnell said it was "inconceivable" that the Senate will have the 67 votes required to remove the president, who has said he would be willing to testify to the inquiry .
Mr Trump has said he did nothing wrong during the call, during which he also asked Ukraine to investigate Mr Biden's son Hunter, who served on the board of a Ukrainian energy company called Burisma.
Lt Col Vindman and Ms Williams admitted his role there could represent a "conflict of interest".