A top adviser to US President Donald Trump says he feared a phone call Mr Trump made to his Ukrainian counterpart - now the focus of an impeachment inquiry - had undermined national security.
Lieutenant Colonel Alexander Vindman is the first person to testify who actually listened in to the call.
He told the investigation on Capitol Hill that he was "concerned" by what was said and "did not think it was proper".
During the conversation on July 25th, Mr Trump asked Ukraine's President Volodymyr Zelenskiy to investigate former US vice-president Joe Biden and his son Hunter Biden, who served on the board of Ukrainian gas company Burisma.
Mr Trump had been withholding US$391m (€351.8m) in security aid to Ukraine.
Mr Zelenskiy has said he only discovered afterwards that aid was being withheld.
Lt Col Vindman, director of European Affairs on the US National Security Council, said that if Ukraine "pursued an investigation into the Bidens and Burisma, it would likely be interpreted as a partisan play which would undoubtedly result in Ukraine losing the bipartisan support it has thus far maintained".
He added: "This would all undermine US national security."
In his opening statement to the inquiry, Lt Col Vindman added: "I was concerned by the call.
"I did not think it was proper to demand that a foreign government investigate a US citizen, and I was worried about the implications for the US government's support of Ukraine."
After the call, Lt Col Vindman said he reported his concerns to the National Security Council's lead lawyer.
The impeachment inquiry was triggered after a complaint from a whistleblower.
Lt Col Vindman denied being the whistleblower or knowing the individual's identity.
He also had concerns just over a fortnight before the call took place.
At a meeting with Ukrainian officials on July 10th in Washington, he said America's ambassador to the EU, Gordon Sondland - a former Trump political donor - told the visitors they needed to "deliver specific investigations in order to secure a meeting with the president".
At that point, then-national security adviser John Bolton cut the meeting short, Mr Vindman said.
He added that Mr Sondland told other US officials in a debriefing after the meeting it was important that any Ukrainian investigations focused on the 2016 election, the Bidens and Burisma.
In prepared remarks, Lt Col Vindman said: "I stated to Ambassador Sondland that his statements were inappropriate, that the request to investigate Biden and his son had nothing to do with national security."
President Trump has questioned Lt Col Vindman's version of events.
He tweeted: "Supposedly, according to the Corrupt Media, the Ukraine call 'concerned' today's Never Trumper witness. Was he on the same call that I was? Can't be possible! Please ask him to read the Transcript of the call. Witch Hunt!"
Supposedly, according to the Corrupt Media, the Ukraine call “concerned” today’s Never Trumper witness. Was he on the same call that I was? Can’t be possible! Please ask him to read the Transcript of the call. Witch Hunt!
— Donald J. Trump (@realDonaldTrump) October 29, 2019