WASHINGTON, Oct. 28 (Xinhua) -- A National Security Council (NSC) official reportedly plans to tell House investigators on Tuesday that he was concerned about U.S. President Donald Trump's July phone call with his Ukrainian counterpart Volodymyr Zelensky, and that he reported his concerns to NSC's lead counsel.

A copy of the opening statement by Army Lieutenant Colonel Alexander Vindman, NSC's top Ukraine expert, was widely reported by U.S. media outlets, before a scheduled deposition for him at Capitol Hill before House panels which was leading an impeachment inquiry into Trump.

"I was concerned by the call. 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," Vindman plans to tell lawmakers, according to his opening statement.

Trump made the phone call with Zelensky on July 25, during which he claimed former Vice President Joe Biden once forced Kiev to fire a prosecutor in order to protect his son Hunter Biden, who served on the board of Burisma, a Ukrainian energy company.

"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," according to Vindman's statement. "This would all undermine U.S. national security. Following the call, I again reported my concerns to NSC's lead counsel."

The Trump-Zelensky phone call is at the heart of the impeachment inquiry initiated last month by House Speaker Nancy Pelosi, after an anonymous whistleblower had raised concerns about the White House's interactions with the Ukrainian government.

Trump was alleged to have abused power by using military aid that Congress approved to pressure Zelensky into investigating Biden, the leading 2020 Democratic presidential candidate in order to help his re-election campaign.

Trump has denied any wrongdoing. Biden has dismissed Trump's accusations.

The White House has said it will not provide documents or witnesses to House investigators because it considered the impeachment inquiry unfair and illegitimate. House Democrats are introducing a resolution to affirm the inquiry and plan to bring it to the floor for a vote later this week.

While a series of Trump administration officials have refused to cooperate despite some receiving a congressional subpoena, other witnesses have testified behind closed doors before House impeachment investigators.

Vindman would be the first person who was actually on the call to testify.

Vindman, who served multiple overseas tours as an infantry officer, began his tenure at the NSC in July 2018.

He was one of the Trump administration's five officials chosen for a U.S. delegation to attend Zelensky's inauguration ceremony in May.