A White House official testified on Tuesday he was so alarmed by hearing U.S. President Donald Trump ask Ukraine's president to investigate a political rival, Democrat Joe Biden, that he reported the matter to a White House lawyer.

Army Lieutenant Colonel Alexander Vindman became the first serving White House official to testify in the Democratic-led House of Representatives inquiry into whether to impeach the Republican president.

Trump Ukraine adviser to testify for impeachment inquiry, Ukraine refuses to get involved

The Ukraine specialist gave a closed-door deposition as Democrats unveiled legislation calling for public hearings and a public report in the impeachment inquiry to blunt Republican criticism that the probe has been conducted with too much secrecy.

While the Democrats met Republican demands for a full House vote on impeachment inquiry procedures, holding public hearings and releasing transcripts taken in secret, senior Republicans rejected the legislation before it was even introduced.

Appearing on Capitol Hill in his military dress uniform, Vindman, a Ukraine-born U.S. citizen and decorated Iraq war combat veteran, became the first person to testify who listened in on the July 25 call at the heart of the Ukraine scandal.

The inquiry has focused on Trump's request of Ukrainian President Volodymyr Zelensky on the call that he investigate Biden, a former vice president, and his son Hunter Biden, who had served on the board of Burisma, a Ukrainian gas company.

Trump also asked Zelensky to investigate a debunked conspiracy theory that Ukraine, not Russia, interfered in the 2016 U.S. election.

Trump made his request after withholding 391 million U.S. dollars in security aid approved by Congress to help Ukraine fight separatists in eastern Ukraine. Zelensky agreed to Trump's requests. The aid was later provided.

"I was concerned by the call," Vindman said in his prepared opening statement to the three House committees conducting the inquiry. "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 the U.S. national security," Vindman added.

Trump has denied any wrongdoing in his dealings with Ukraine and has called the impeachment probe politically motivated.

Vindman's testimony was some of the most damaging to date for Trump, who faces the possibility of impeachment as he prepares to run for re-election. Biden is a leading contender for the 2020 Democratic presidential nomination to face him.

Federal law prohibits candidates from accepting foreign help in an election.

Vindman, appearing after receiving a subpoena from lawmakers despite the Trump administration policy of not cooperating with the impeachment inquiry, recounted listening in on the call in the White House Situation Room with colleagues from the National Security Council and U.S. Vice President Mike Pence's office.

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

Vindman said earlier in July he had also reported concerns about previous pressure by the administration on Ukraine to carry out politically motivated investigations to the lawyer.