The White House attacked its own top Ukraine official as he testified to an impeachment hearing that a phone call made by the president was "improper" and had left him in "shock".
Lt Col Alexander Vindman told Congress that President Donald Trump made "inappropriate" political demands of the Ukrainian president.
"I couldn't believe what I was hearing," Col Vindman said.
The hearings are investigating whether Mr Trump abused his presidential power.
A decorated Iraq war veteran who serves in a senior role on the US National Security Council (NSC), Col Vindman testified before the House on Tuesday in his Army dress uniform.
As he described his reaction to hearing a call between President Trump and his Ukranian counterpart, President Volodymyr Zelensky, Col Vindman came under attack by the official White House Twitter account, which posted a critical quote from his former boss on the NSC questioning his judgement.
Republican congressmen pressed Col Vindman on the remark and questioned his loyalty to the US - asking about instances in which Ukrainian officials approached him about becoming the country's defence minister.
"Every single time I dismissed it," Col Vindman said. "I'm an American."
The impeachment hearings are an effort by Democrats to establish whether Mr Trump withheld US military aid to Ukraine in order to pressure the country's new leader, Mr Zelensky, into announcing a corruption inquiry into Joe Biden, Mr Trump's leading Democratic rival for the US presidency.
If the president were to be impeached by a majority vote in the House of Representatives, he would face a trial in the Senate which could remove him from office, although the Republican controlled chamber would be unlikely to vote against him.
White House goes on the attack
Col Vindman told the US House intelligence committee that he had been concerned by the president's demands to investigate Mr Biden.
"It was probably an element of shock that maybe, in certain regards, my worst fear of how our Ukrainian policy could play out was playing out," he said.
"It was improper for the president to request - to demand - an investigation into a political opponent, especially [from] a foreign power where there is at best dubious belief that this would be a completely impartial investigation and that this would have significant implications if it became public knowledge," he told the committee.
He had reported the "inappropriate" discussion to NSC lawyers "out of a sense of duty".
The official Twitter account of the White House was used throughout the hearing to attack Col Vindman and the inquiry in real time, retweeting hashtags including #ShamImpeachment and #ParodyImpeachment.
The Trump administration's use of a taxpayer-funded account to attack opponents has drawn criticism in the past.
Col Vindman was among the US officials who listened in on the 25 July call between the two leaders. He is a decorated Iraq war veteran who was born in Ukraine; his family moved from the Soviet Union to the US 40 years ago.
Mr Trump similarly used his own personal Twitter account to attack the former US ambassador to Ukraine, Marie Yovanovitch, during her testimony to the impeachment inquiry last week.
Did Vindman show uncertainty or humanity?
If what Vindman said was important, how he said it in the public hearings also mattered.
Behind closed doors, veteran ambassadors Bill Taylor and Marie Yovanovitch were reportedly smooth while Vindman was halting and nervous. Those observations have been confirmed by their public testimony - when Vindman delivered his opening statement, his hands trembled slightly. He occasionally stumbled over his words.
Republicans could paint that as weakness or uncertainty, but it might also be seen by Americans as giving his testimony a touch of humanity - particularly when paired with the emotional closing words to Vindman's opening statement.
Vindman offered reassurance to his father, who brought his children to the US from the Soviet Union, that he was sitting in the US Capitol and would be "fine for telling the truth".
Toward the end of Vindman's appearance, he was asked by a Democratic congressman to read that line again - and then added why he's not afraid of testifying today. "Because this is America," he said. "This is the country I have served and defended... and here, right matters."
Vindman's testimony will only add to the contentious debate among Democrats and Republicans over who is right and exactly what the truth is.
Who else testified on Tuesday?
The other witness in the morning session was Jennifer Williams, a foreign service adviser to Vice-President Mike Pence.
She said Mr Trump's reference to Mr Biden in the 25 July call with Mr Zelensky had been "unusual" because it delved into domestic US politics.
"The reference to Biden sounded political to me," she said.
The inquiry's afternoon session heard from former NSC official Tim Morrison and Kurt Volker, the former US special representative to Ukraine.
Mr Volker told the hearing that President Trump "had a deeply rooted negative view on Ukraine rooted in the past" and despite "positive news and recommendations" being conveyed about Ukraine's new president "he was clearly receiving other information from other sources, including Mayor Giuliani, that was more negative, causing him to retain this negative view".
He added that the allegations against Mr Biden were "self-serving and not credible".
Mr Morrison - who resigned from his position some weeks ago - says he felt no pressure to quit and feared no retaliation.
In his opening statement he said he did not know the identity of a whistleblower whose report helped start the inquiry.
What is the inquiry about?
Mr Trump is facing a process that could eventually see him removed from office.
The inquiry is trying to establish whether or not he improperly sought help from Ukraine to boost his chances of re-election in 2020.
Things are still at an early stage. The first public hearings started last week in the lower house of Congress, the House of Representatives, controlled by the Democrats. President Trump, who is a Republican, strongly denies any wrongdoing.
Tuesday is the third day of televised impeachment hearings.
Depending on what happens in the next few weeks, Mr Trump could end up facing impeachment.
Learn more about the impeachment inquiry
- GO DEEPER: Here's a 100, 300 and 800-word summary of the story
- WHAT'S IMPEACHMENT? A political process to remove a president
- VIEW FROM TRUMP COUNTRY: Hear from residents of a Virginia town
- ON THE DOORSTEP: A Democrat sells impeachment to voters