Four current and former US officials testify in public impeachment hearings as the inquiry intensifies.

The US impeachment inquiry of Donald Trump kicks off its second week of public hearings on Tuesday with four current and former United States officials set to testify.

The House Intelligence Committee will first hear from Jennifer Williams, a special adviser to Vice President Mike Pence, and US Army Lieutenant Colonel Alexander Vindman, the National Security Council’s top Ukraine expert. They listened to the call between Trump and his Ukrainian counterpart, Volodymyr Zelenskyy, that prompted a whistle-blower complaint and eventually led to the impeachment inquiry.

A second hearing will follow with Kurt Volker, the former US special envoy for Ukraine, and Tim Morrison, a former National Security Council Russia expert.

The scheduled testimony comes after a dramatic first week of hearings that included unheard testimony, and accusations of witness intimidation as Trump attacked one of the witnesses on Twitter as a hearing got under way.

The Democratic-led House impeachment inquiry is examining whether Trump sought to leverage Zelenskyy’s desire for a White House meeting and nearly $400m in frozen US security aid that Ukrainian forces needed to battle Russia-backed separatists in return for gaining political advantage over former Vice President and top presidential hopeful Joe Biden.

President Donald Trump meets with Ukrainian President Volodymyr Zelenskyy at the InterContinental Barclay New York hotel during the United Nations General Assembly [File: Evan Vucci/AP Photo]

At the inquiry’s core is a July 25 telephone call in which Trump pressed Zelenskyy to investigate Biden and his son, who had served as a member of the board of Burisma, a Ukrainian energy firm, and a discredited conspiracy theory that it was Ukraine – not Russia as determined by US intelligence agencies – that interfered in the 2016 US election.

Trump has repeatedly denied wrongdoing and raged on Twitter against what he calls “a witch-hunt” and a “hoax”. He said on Monday he might testify before the House Intelligence Committee, which is leading the inquiry. Democrats responded with scepticism.

Vindman: ‘No ambiguity ‘ about investigations requests

Vindman and Williams were among a handful of US officials who listened to the July 25 call.

In his closed-door testimony, Vindman told House investigators he raised concerns about the call with White House lawyers.

“I was concerned by the call,” Vindman said, according to a transcript of the testimony. “I did not think it was proper to demand that a foreign government investigate a US citizen, and I was worried about the implications for the US government’s support of Ukraine.”

“I realised 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 US national security,” he said.

Vindman, a military officer at the National Security Council, departs a closed-door meeting after testifying as part of the House impeachment inquiry into President Donald Trump [File: Patrick Semansky/AP Photo]

Vindman also said that Gordon Sondland, the ambassador to the European Union, told Ukrainian officials they would need to investigate the Bidens if they hoped to have a coveted meeting with Trump, according to the transcript.

“When investigators pushed if the requested investigation was specifically into the Bidens, Vindman responded: “My visceral reaction to what was being called for suggested that it was explicit.”

He added: “There was no ambiguity.”

Vindman also told the House panel that a memo of the July 25 call between Trump and Ukraine’s leader at the centre of the probe was edited to remove a specific reference to Burisma. Vindman said the rough transcript was edited to replace Burisma with “the company”.

Williams: Some of Trump’s comments ‘inappropriate’

Williams testified behind closed doors this month that some of Trump’s comments were “inappropriate”.

The president assailed her on Twitter on Sunday as a “Never Trumper” who should “work out a better presidential attack”.

Williams, special adviser for Europe and Russia in the Office of US Vice President Mike Pence, arrives on Capitol Hill [File: Tom Brenner/Reuters]

Williams, a longtime State Department official who is detailed to Pence’s national security team, said she too had concerns during the phone call, which the aides monitored as is standard practice.

When the White House produced a rough transcript later that day, she put it in the vice president’s briefing materials. “I just don’t know if he read it,” Williams testified in a closed-door House interview.

Volker: ‘Biden was never a topic of discussion’

Volker and Morrison are the first two witnesses sought by Republicans to testify in the public hearings.

Volker won praise from Republicans after he testified behind closed doors last month that he did not know of any effort by Trump to press Ukraine to investigate Biden.

Republicans called him “impressive” and said his testimony did not match the Democrats’ pro-impeachment narrative.

“Vice President Biden was never a topic of discussion,” he said in his October 3 closed-door testimony.

Volker, a former special envoy to Ukraine, arrives for a closed-door interview with House investigators as House Democrats proceed with the impeachment inquiry of President Donald Trump [File: Scott Applewhite/AP Photo]

But other witnesses have identified Volker as one of the “Three Amigos”, along with Energy Secretary Rick Perry and diplomat Sondland, who were tasked by Trump to obtain Zelenskyy’s commitment to probing Biden and his son, Hunter.

Volker resigned from his position as the special envoy for Ukraine at the end of September.

Morrison concerned about ‘potential leak’

Morrison, meanwhile, told investigators he was concerned that “a potential leak” of the White House’s memorandum of the conversation “would play out in Washington’s polarised environment”, according to his prepared remarks for his closed-door testimony.

Morrison said he feared Trump’s call with Zelenskyy “would affect the bipartisan support our Ukrainian partners currently experience in Congress” and “would affect the Ukrainian perceptions of the US-Ukraine relationship”.

Morrison, special assistant to the president and senior director for Europe and Russia at the National Security Council, returns to testify at a closed-door deposition as part of the impeachment inquiry [File: Siphiwe Sibeko/Reuters]

Tuesday’s scheduled testimony kicks off a week of hearings, with several more current and former US officials set to testify as the impeachment inquiry deepens.

The investigation could lead the House to approve formal charges – known as articles of impeachment – against Trump that would be sent to the Republican-controlled Senate for a trial that could result in his removal from office.

At the moment, that outcome is doubtful as few Republican senators have broken with Trump.

Only two US presidents, Bill Clinton and Andrew Johnson, have been impeached, but neither was removed from office. President Richard Nixon faced impeachment and resigned in 1974 over the Watergate scandal.