Lt. Col. Alexander S. Vindman, the top Ukraine expert on the White House National Security Council, is set to testify privately to congressional investigators on Tuesday in the impeachment inquiry.

According to his opening statement, first reported by The New York Times, he will testify that he felt a "sense of duty" to speak out about President Donald Trump's dealings with Ukraine, which he feared would "undermine US national security."

Vindman's testimony will be the first in the impeachment inquiry from a White House official who listened in on Trump's July 25 phone call with Ukrainian President Volodymyr Zelensky. The call, which sparked the impeachment proceedings, featured Trump pressuring Zelensky to investigate former Vice President Joe Biden and his son Hunter.

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A White House national security aide is set to testify before congressional investigators Tuesday as part of the House impeachment inquiry and is expected to tell them that he twice reported concerns that President Donald Trump's actions were "undermining US national security."

Lt. Col. Alexander S. Vindman, the top Ukraine expert on the White House National Security Council, will testify privately to the House Intelligence, Foreign Affairs, and Oversight and Reform committees that he twice registered concerns about Trump and his inner circle's dealings with Ukraine, according to a draft of his opening statement first obtained by The New York Times on Monday.

Vindman's testimony is set to be the first from a White House official who listened in on Trump's July 25 phone call with Ukrainian President Volodymyr Zelensky, a call in which Trump sought to pressure Zelensky to investigate former Vice President Joe Biden and his son Hunter.

The call was the subject of a whistleblower complaint filed in August and has since become the launchpad for Democrats in their impeachment inquiry. The White House has said it will not to cooperate with the impeachment inquiry.

Vindman said in his statement that he felt a "sense of duty" to speak out against Trump's actions, which he recalled fearing would "undermine US national security."

July 10: Vindman reported that he was concerned by 'inappropriate' comments made by US Ambassador Gordon Sondland

Gordon Sondland, the US ambassador to the European Union, at a press conference at the US Embassy to Romania in Bucharest on September 5. DANIEL MIHAILESCU/AFP/Getty Images

Vindman flagged conversations between the White House and Ukrainian officials as early as July 10.

In his statement, posted online by Politico, Vindman recalled a meeting he participated in involving Oleksandr Danylyuk, the secretary of the National Security and Defense Council for Ukraine, and other White House officials including the national security adviser, John Bolton, and the US ambassadors Kurt Volker and Gordon Sondland.

Vindman said that Sondland, the ambassador to the European Union, "started to speak about Ukraine delivering specific investigations" to secure a meeting with Trump, "at which time Ambassador Bolton cut the meeting short."

He said Sondland emphasized the importance of the investigations again during a debriefing. Following the debrief, Vindman reported concerns about Sondland's comments, which he called "inappropriate," to the NSC's lead counsel, along with Fiona Hill, Trump's former Russia adviser who recently testified in the impeachment inquiry.

Bill Taylor, the US's acting ambassador to Ukraine, gave testimony last week that also discussed Sondland's comments while painting a picture of Trump withholding military aid to Ukraine in exchange for politically motivated investigations.

July 25: Vindman listened in on Trump's July 25 call with Zelensky and reported that Trump's actions would 'undermine US national security'

Trump with Ukrainian President Volodymyr Zelensky on the sidelines of the 74th session of the United Nations General Assembly in New York City. Jonathan Ernst/Reuters

Vindman also listened in on the July 25 call between Zelensky and Trump and said he was concerned by it.

"I did not think it was proper to demand that a foreign government investigate a US citizen," Vindman wrote in his statement, "and I was worried about the implications for the US government's support of Ukraine."

"I realized that if Ukraine pursued an investigation into the Bidens ... it would likely be interpreted as a partisan play, which would undoubtedly result in Ukraine losing the bipartisan support it has thus far maintained."

Following the call, he again reported his concerns to the NSC's lead counsel.

Vindman plans to testify that he is not the whistleblower who filed the August complaint against Trump and does not know the person's identity. His account of the phone call, however, corroborates the account given by the anonymous intelligence official behind the complaint.

Vindman to speak about 'outside influencers' who painted a false narrative of Ukraine

Trump with Rudy Giuliani at the Trump National Golf Club Bedminster clubhouse in Bedminster, New Jersey, in 2016. AP

Vindman, an Army veteran who was awarded a Purple Heart, served in the US embassies in Moscow and the Ukrainian capital of Kyiv but said he never had direct contact or communications with Trump. He said he became aware of "outside influencers promoting a false narrative of Ukraine" in the spring of 2019.

"This narrative was harmful to US government policy," he wrote.

Last week, The Washington Post reported that Russian President Vladimir Putin and Hungarian Prime Minister Viktor Orban had urged Trump to adopt a "hostile view" of Ukraine that helped reinforce the notion that the country was "hopelessly corrupt." Current and former US officials told The Post that Trump's personal lawyer Rudy Giuliani also reinforced negative views of Ukraine.