A top US official restricted access to the summary of Donald Trump's Ukraine call that triggered an impeachment probe, fearing it would damage the president, testimony released on Saturday showed.White House Ukraine expert Timothy Morrison told House investigators he knew immediately how sensitive the call was when he heard Trump press Ukraine's President Volodymyr Zelensky to investigate former US vice president Joe Biden.READ | Trump attacks US diplomat as she testifies in impeachment probeMorrison said in his October 31 deposition that he had asked National Security Council lawyers to review the call because he thought it would be "damaging" if it leaked."I recommended to them that we restrict access to the package... that we did not need quite so many people to have access to the package," said Morrison, who left the NSC a day before testifying.The conversation, which took place on July 25 while the White House was withholding military aid to Ukraine, is central to the House impeachment inquiry into Trump.'Alarm bells'READ | Trump ally Roger Stone guilty of lying to CongressDemocrats leading the probe say the call summary shows the president abused his office by bullying a vulnerable ally into interfering on his behalf in the 2020 US election.The investigation threatens to make Trump the third US president to be impeached, although the Republican-controlled Senate would need to convict him to remove him from office."The testimony released today shows that President Trump's July 25 phone call with Ukrainian President Zelensky immediately set off alarm bells throughout the White House," Adam Schiff, the congressman leading the inquiry, said in a statement released jointly with other senior Democrats.Morrison said however the summary had been placed on a highly classified system due to a "mistake" by White House staff - casting doubt on allegations that it was squirreled away as part of a deliberate cover-up.He also broke with other witnesses who have spoken out against the call, telling House investigators he heard nothing wrong.House investigators also released testimony on Saturday from Jennifer Williams, an aide to Vice President Mike Pence, who told lawmakers Trump's push for Ukraine to open investigations was "unusual and inappropriate".Williams listened in on the July 25 call and said the requests appeared to have been made to further Trump's "personal political agenda" rather than US foreign policy goals."I guess for me it shed some light on possible other motivations behind a security assistance hold," she added.The depositions were made public as another official arrived on Capitol Hill to answer questions on the White House's withholding of nearly $400m in congressionally allocated military aid for Ukraine.'Get over it'Mark Sandy was the first staffer from the White House Office of Management and Budget (OMB) to testify, rejecting a Trump administration directive not to co-operate.OMB acting director Russell Vought and acting White House chief of staff Mick Mulvaney, who still has the title of OMB director, defied congressional subpoenas to appear.Previous witnesses have testified that the money was delayed as part of a wider extortion plot to extract a public announcement from Zelensky that Biden was being investigated.The White House says Trump delayed the cash because of broad concerns over Ukrainian corruption but Mulvaney undermined this defence when he told reporters in October that there was indeed a "quid pro quo" of aid for investigations."That's why we held up the money," he said. "Get over it."