Jennifer Williams, a foreign policy aide to Mr. Pence who was listening to the call on July 25, testified that Trump's insistence that Ukraine carry out politically sensitive investigations “struck me as unusual and inappropriate.”

The phone call between U.S. President Donald Trump and his Ukrainian counterpart at the center of Congress' impeachment investigation was ”inappropriate,” an aide to Vice President Mike Pence told lawmakers, according to a transcript released on Saturday.

Jennifer Williams, a foreign policy aide to Mr. Pence who was listening to the call on July 25, testified that Trump's insistence that Ukraine carry out politically sensitive investigations “struck me as unusual and inappropriate.”

She said the discussion was “more political in nature” than phone calls with other foreign leaders, and included what she viewed as specific references to the president's “personal political agenda.”

Mr. Trump's call is at the heart of the Democratic-led inquiry into whether the Republican president misused U.S. foreign policy to undermine former Vice President Joe Biden, one of his potential opponents in the 2020 election.

The House of Representatives on Saturday also released a transcript of an earlier closed-door deposition by Tim Morrison, a former White House aide with the National Security Council focusing on Europe and Russia policy, who was also on the call.

Mr. Morrison, who resigned a day before his deposition last month, and Ms. Williams both expressed concerns about Mr. Trump's remarks to Ukrainian President Volodymyr Zelenskiy. Ms. Williams and Mr. Morrison are scheduled to testify publicly next week.

“I was not comfortable with any idea that President Zelenskiy should allow himself to be involved in our politics,” Morrison told lawmakers. He also said found Zelenskiy's tone in the call with Trump to be “obsequious.”

Mr. Morrison declined to say he thought the call was illegal or improper, stressing instead that he thought it would leak, damaging relations with Ukraine. He said he only learned later that the aid to Ukraine was conditioned on the investigations.

In the first public hearing last week, acting ambassador to Ukraine William Taylor pointed to Trump's keen interest in getting the eastern European ally to investigate Biden and reiterated his understanding that $391 million in U.S. security aid was withheld from Kiev unless it cooperated.

Mr. Morrison said he had reviewed Mr. Taylor's testimony and did not dispute it on any significant points.

House investigators on Saturday also heard closed-door testimony from a White House budget official about the holdup of military aid to Ukraine. Mark Sandy, a career official of the Office of Management and Budget, was the first person from OMB to testify before the inquiry after three political appointees defied congressional subpoenas to appear.

Mr. Sandy spoke about how unusual it was to have a political appointee come in and place a hold on military aid to Ukraine, CNN quoted a source familiar with the testimony as saying.

Democratic Representative Jamie Raskin, a member of the Judiciary and Oversight Committees, said Mr. Sandy was brought in to shed light on whether military aid was held up for political reasons.

“This is a technical part of our investigation,” Mr. Raskin told reporters outside the interview room.

“We want to know exactly how the president translated his political objective to shake down the Ukrainian government for the favors he wanted [into] the budget process.”

Holmes’ Testimony

On Friday, Mr. Trump used Twitter to criticize a former U.S. ambassador to Ukraine, Marie Yovanovitch, while she was testifying to an impeachment hearing, an extraordinary moment that Democrats said amounted to witness intimidation.

Mr. Trump defended his attack on Ms. Yovanovitch, saying he had the right to free speech.

Ms. Yovanovitch, a career diplomat, testified on the second day of televised impeachment hearings, which will resume next week.

While Ms. Yovanovitch's testimony dominated headlines on Friday, a closed-door deposition later in the day with David Holmes, a U.S. embassy official in Kiev, could prove more consequential.

Mr. Holmes told lawmakers he overheard a phone call between Mr. Trump and Gordon Sondland, the U.S. ambassador to the European Union, in which the president inquired about Ukraine's willingness to carry out investigations of Mr. Biden and his son.

The phone call occurred on July 26, one day after the now-infamous phone conversation between Mr. Trump and Mr. Zelenskiy.

“So, he's gonna do the investigation?” Mr. Trump asked Mr. Sondland, referring to Mr. Zelenskiy, according to Mr. Holmes' testimony.

“He's gonna do it,” replied Mr. Sondland, adding the Ukrainian president would do “anything you ask him to,” according to Mr. Holmes.

The testimony by Mr. Holmes, an aide to Mr. Taylor, ties Mr. Trump more directly to a pressure campaign in Ukraine to investigate the Bidens led by Trump's personal lawyer Rudy Giuliani.

Mr. Holmes' statement appears to contradict Mr. Sondland's previous sworn testimony about his interactions with Mr. Trump, in which he did not mention his July 26 phone call with the president.

Mr. Sondland, who has already revised his testimony once, is scheduled to testify publicly on Wednesday.

Mr. Morrison said he understood that Mr. Sondland and Mr. Trump had spoken “half a dozen times” between mid-July and mid-September, adding that he, National Security Adviser John Bolton and other officials viewed Mr. Sondland's involvement as “a problem.”

Representative Eric Swalwell, a Democrat on the House Intelligence Committee, told reporters on Saturday that House leaders were considering bringing Mr. Holmes in for a public hearing.