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

Jennifer Williams, 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 former U.S. ambassador to Ukraine also testified on Friday

Yovanovitch 'intimidated' as Trump live-tweets impeachment hearing

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

As did Williams, Morrison told lawmakers he had concerns about Trump's remarks to Ukrainian President Volodymyr Zelenskiy. Williams and 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," said Morrison, who also was on the call.

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 U.S. Vice President Joe Biden, one of his potential opponents in the 2020 election.

Morrison declined to say he thought the call was illegal or improper, stressing instead that he thought it would leak and damage relations with Ukraine.

In a disclosure that drew the most attention 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.

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