A senior White House official has told the impeachment inquiry that Donald Trump's "demand" for Ukraine to launch an investigation into his political rival Joe Biden was "improper" and "inappropriate".
Appearing before the congressional committee leading the inquiry on Tuesday, Lieutenant Colonel Alexander Vindman, Mr Trump's top Ukraine adviser, described his shock as he listened to the US president's call with the Ukrainian president Volodymyr Zelenskiy on July 25.
Lt Col Vindman said that the request for the Biden probe amounted to a "demand", given the power imbalance between the US president and the Ukrainian president, who had just been elected and was seeking a meeting with Mr Trump.
Lt Col Vindman also recounted that after the call was finished he went to the White House's lawyers to raise his concerns about what Mr Trump had done, fearing there were "significant national security implications" from the president's actions.
Ukraine launching such an investigation would "undoubtedly result in Ukraine losing bipartisan support, undermine US national security, and advance Russia's strategic objectives in the region", Lt Col Vindman said in his opening statement.
The testimony came on the third day of public impeachment hearings on Capitol Hill. Mr Trump insists that the July 25 call was "perfect".
A transcript of the call later released shows that the president asked Mr Zelenskiy for a probe into Mr Biden, the former US vice president and frontrunner as Democratic presidential candidate for the 2020 election, and his son Hunter Biden, who once sat on the board of a Ukrainian gas company.
Mr Trump also sought an investigation into unfounded claims that Ukraine interfered in the 2016 presidential election during the call.
Lt Col Vindman was joined in giving testimony by Jennifer Williams, a State Department official who works as a Russia and Ukraine adviser to Mike Pence, the US vice president.
Ms Williams, who along with Lt Col Vindman sat in the White House situation room and listened to the July 25 call, also described her surprise at how the conversation played out.
Ms Williams told the impeachment inquiry that she found the call "unusual" because "it involved what appeared to be a domestic political matter" - an apparent reference to the investigations that Mr Trump was seeking.
During questioning, both Lt Col Vindman and Ms Williams said that they were not aware of any credible allegation that Mr Biden had done anything wrong in his actions towards Ukraine. Mr Biden led America's Ukraine policy while vice president under Barack Obama.
Taken together, the testimony makes it harder for Mr Trump and his Republican allies to wave away the scandal that has erupted over the president's push to secure an investigation into the Democrat he may have to defeat to win re-election.
Both US officials who gave evidence on Tuesday have spent more than a decade in public service.
Lt Col Vindman, whose father fled the Soviet Union for America 40 years ago, was injured during military service in Iraq and was awarded a Purple Heart for his actions. He appeared before the House Intelligence Committee on Tuesday in his military uniform.
Ms Williams, who was derided as a 'Never Trumper' by the president ahead of her testimony, works for Mr Pence. She described securing the job as "the greatest honour of my career".
During his opening testimony, Lt Col Vindman also appeared to direct criticism towards Mr Trump as he issued a fierce rebuke of those who have attacked impeachment witnesses thus far.
Lt Col Vindman said: "I want to state that the vile character attacks on these distinguished and honourable public servants is reprehensible.
"It is natural to disagree and engage in spirited debate, this has been our custom since the time of our Founding Fathers, but we are better than callow and cowardly attacks."
He did not name Mr Trump specifically, but the president has faced a backlash after tweeting attacks against a number of witnesses - some US officials still working in his administration - who have agreed to give evidence.
During Tuesday’s hearing Democrat and Republican committee members and their lawyers attempted to tease out comments from the witnesses which could bolster their cases in the impeachment battle which threatens Mr Trump’s presidency.
Democrats asked Lt Col Vindman to recount how he had heard the Ukrainian president explicitly mention the word "Burisma" - the Ukrainian gas company which Hunter Biden worked for - during the July 25 call, but that it was not included in the transcript.
Lt Col Vindman confirmed that he had flagged up the missing word, which gives a more direct link between the Ukrainian president and a Biden probe, after he read a draft of the call transcript. However he said he thought the error was an honest mistake by note-takers.
The Republican lawyer on the committee questioned whether Mr Trump's comments during the call did really amount to a "demand", suggesting instead that the president’s words were ambiguous.
He also attempted to establish Mr Trump's concern for Ukraine corruption more generally - part of a push by the Republicans to frame the president's investigation demands as motivated by genuine concerns about wrongdoing rather than personal political reasons.
Ms Williams's testimony also offered insights into what Mr Pence, her boss, may have known about the president's attempt to secure the Biden and Ukraine 2016 meddling investigations.
Ms Williams said that she added a mention of the July 25 call after it happened into Mr Pence's daily briefing book and included a hard copy of the rough transcript. She said she did not know if Mr Pence read the transcript.
Ms Williams also described how Mr Pence initially expressed enthusiasm for attending Mr Zelenskiy's inauguration in Ukraine but that she later was told Mr Trump "had decided" that Mr Pence should not make the trip.
The evidence is significant as Mr Trump is facing claims that he held back engagement with the Ukrainian president in an attempt to secure the investigations he was seeking.
Mr Trump has denied any wrongdoing.