Senior US officials have told impeachment investigators in Congress they were concerned by President Donald Trump's effort to get Ukraine to investigate a political rival, with one White House official calling it a "shock."
The third day of impeachment hearings in the House of Representatives marked the first time White Office officials publicly expressed misgivings about a pressure campaign now threatening Trump's presidency.
The White House's top Ukraine expert, wearing his Army uniform, said Trump had made an "improper" demand of Ukraine President Volodymyr Zelenskiy in a July 25 phone call that has become the centerpiece of the Democratic-led impeachment probe of the Republican president.
"Frankly, I couldn't believe what I was hearing. 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," Army Lieutenant Colonel Alex Vindman said on Tuesday.
Two other senior White House aides, Jennifer Williams and Tim Morrison, also said they were concerned by the political nature of that phone call.
Williams said Trump's call with Zelenskiy was unusual and inappropriate because "it involved discussion of what appeared to be a domestic political matter."
Morrison said he did not see anything improper in the call but was concerned that its contents could leak, hurting bipartisan support for Ukraine. "I wanted access to be restricted," he said.
During that call, Trump asked Zelenskiy to carry out two investigations that would benefit him politically, including one targeting Joe Biden, the former vice president who is a leading Democratic presidential contender to face Trump in next year's election, and his son Hunter Biden.
The other involved a debunked conspiracy theory embraced by some Trump allies that Ukraine, not Russia, interfered in the 2016 US election.
Kurt Volker, a former US envoy to Ukraine, said allegations of corruption involving Biden and his son, who was a director of Ukrainian energy company Burisma, were "not credible."
Ahead of the July call, Trump had frozen $US391 million in US security aid approved by Congress to help Ukraine combat Russia-backed separatists in the eastern part of the country.
Volker testified that Trump described Ukraine as "a corrupt country, full of terrible people."
"He said they 'tried to take me down,'" Volker added.
Volker said he did not know that a request to tackle corruption in Ukraine and investigate Burisma, a natural gas company, was effectively a request to investigate Biden.
"In retrospect, I should have seen that connection differently, and had I done so, I would have raised my own objections," he said.
Trump has attacked both Williams and Vindman on Twitter as "Never Trump" witnesses.
Democratic Representative Jim Himes asked Vindman: "Lieutenant Colonel, would you call yourself a Never Trumper?"
"I'd call myself never partisan," Vindman replied.
The president's son Donald Trump Jr. assailed Vindman in a Twitter post as "a low level partisan bureaucrat and nothing more."
A US official said Vindman and his family might be moved to a military base because of security threats.
Vindman said he was not worried that he would suffer reprisals for speaking out.
"This is a country I have served and defended, that all of my brothers have served - and here, right matters," he said, drawing applause.
The investigation could lead the House to approve formal charges against Trump - called articles of impeachment - that would be sent to the Republican-controlled Senate for a trial on whether to remove him from office. Few Republican senators have broken with Trump.
Senate Majority Leader Mitch McConnell said on Tuesday it was "inconceivable" that two-thirds of the Republican-controlled chamber would vote to convict Trump.
Australian Associated Press