President Donald Trump's top adviser on Ukraine is set to testify on Tuesday that he told a government lawyer about concerns that U.S. national security could be undermined after a phone call between Trump and Ukraine's president.

Army Lieutenant Colonel Alexander Vindman will be the first current White House official to testify behind closed doors in the impeachment inquiry led by the House of Representatives Intelligence, Foreign Affairs and Oversight committees.

Vindman, director of European affairs on the National Security Council, listened in on the July 25 call, in which Trump asked Ukrainian President Volodymyr Zelenskiy to investigate former Vice President Joe Biden, a Democratic political rival, and his son Hunter Biden, who had served on the board of Ukrainian gas company Burisma.

The call prompted a complaint from an intelligence community whistleblower that led to the inquiry.

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In his prepared opening statement, Vindman recounted listening in on the call. "I was concerned by the call," he said. "I did not think it was proper to demand that a foreign government investigate a U.S. citizen, and I was worried about the implications for the U.S. government's support of Ukraine."

"I realized 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. This would all undermine U.S. national security."

After the call, Vindman said he reported his concerns to the NSC's lead counsel.

According to Vindman, the importance of the Ukraine launching an investigation into the Bidens and Burisma was also emphasized by U.S. Ambassador to the European Union Gordon Sondland in a meeting after a visit by a Ukrainian security official to Washington on July 10.

"I stated to Amb. Sondland that his statements were inappropriate, that the request to investigate Biden and his son had nothing to do with national security," Vindman said.

Vindman stated he became aware of a shadow Ukraine policy promoted by "outside influencers" in the spring of 2019. In his role as a director on the NSC, he provided readouts of relevant meetings and communications on Ukraine and other countries in his portfolio.

Trump made his request to Zelenskiy after withholding 391 million U.S. dollars in security aid approved by Congress to help Ukraine fight Russian-backed separatists in the eastern part of the former Soviet republic.

Federal law prohibits candidates from accepting foreign help in an election. Biden is a leading candidate for the Democratic presidential nomination to face Trump in the November 2020 election.

Ukrainian officials will not testify in the U.S. Congress in the impeachment inquiry because Kiev does not want to get involved in the internal affairs of another country, Ukraine's foreign minister said on Tuesday.

"We don't have anything to do with this at all," Vadym Prystaiko told reporters on the sidelines of an investment conference in the port city of Mariupol. "We won't go there, we won't comment."

Prystaiko said relations between Kiev and Washington had not changed since the start of the impeachment row and that it was normal for the two countries to use formal and informal channels to conduct diplomacy.

"There are channels that are especially separated from the official ones, but this does not mean that the official ones don't know what is happening," he said.

He also said Ukraine needed to receive official requests from the United States in order to open investigations.

Asked whether Ukraine had received such a request to investigate Burisma, he said that question should go to the General Prosecutor's office.

(With input from Reuters)

(Cover: Lt. Col. Alexander Vindman, director for European Affairs at the National Security Council, arrives to testify on Capitol Hill in Washington, U.S., October 29, 2019. /Reuters Photo)