President-elect Donald Trump meets with former New York City Mayor Rudy Giuliani at the clubhouse of the Trump National Golf Club in Bedminster, New Jersey.(Photo:VCG)

A top US diplomat told congressional investigators that President Donald Trump's personal attorney Rudy Giuliani conducted a "campaign full of lies" against the US ambassador to Ukraine before she was recalled from her post, according to a transcript of his testimony released on Thursday.

George Kent, a deputy assistant secretary of state, told the Trump impeachment inquiry that he was also subject to attacks by Giuliani but was told to "keep my head down" by a senior State Department official.

The Democratic-led inquiry in the House of Representatives is focused on a July 25 phone call in which Trump asked Ukrainian President Volodymyr Zelensky to investigate Joe Biden, a former vice president and now a leading Democratic rival in the November 2020 presidential election.

Giuliani, is central to the inquiry and he has been mentioned frequently in testimony by State Department diplomats who have painted a picture of the former New York City mayor running a shadow US policy toward Ukraine to pressure it to carry out a corruption investigation into Biden and his son, who worked for a Ukrainian gas company.

Kent mentioned Giuliani 73 times in his testimony to lawmakers which was given behind closed doors session on October 15 but only released on Thursday.

Marie Yovanovitch, the former US ambassador to Ukraine, was abruptly pulled from her post in May. Kent said Giuliani conducted a smear campaign against the envoy. "His assertions and allegations against former Ambassador Yovanovitch were without basis, untrue, period," Kent testified.

"Mr. Giuliani, at that point, had been carrying on a campaign for several months full of lies and incorrect information Ambassador Yovanovitch, so this was a continuation of his campaign of lies," Kent said.

Neither Giuliani nor a lawyer for him immediately responded to requests for comment on Kent's testimony.

Kent said Ukrainian officials understood when they met with Giuliani that he was not a regular private citizen and understood he represented Trump.

"Giuliani was not consulting with the State Department about what he was doing in the first half of 2019. And to the best of my knowledge, he's never suggested that he was promoting US policy," Kent said.

For nearly a year, Giuliani has pursued unsubstantiated allegations that Biden pushed to fire a Ukrainian prosecutor to stop him from investigating Burisma, the Ukrainian energy company, where Biden's son Hunter served on the board.

Giuliani also told Reuters he played a role in the effort to remove Yovanovitch.

Star witnesses

Democrats have been releasing transcripts of the closed-door interviews as they prepare for public hearings in Congress next week. Kent is among the three US diplomats who will serve as star witnesses.

Lawmakers are trying to determine whether Trump froze 391 million US dollars in US security assistance for Ukraine to put pressure on Zelenskiy to conduct the investigation, thus misusing US foreign policy for his personal gain.

Trump's defenders say there is no evidence of him and the Ukrainian president engaging in a "quid pro quo" - exchanging a favor for a favor - because the aid to Ukraine was released and Zelensky never explicitly promised anything.

A quid pro quo is not necessary to prove high crimes or misdemeanors, which is the standard the US Constitution requires for the impeachment of a president.

Kent and William Taylor, the top US diplomat in Ukraine will testify on November 13. Yovanovitch will testify on November 15.

If the Democratic-controlled House votes to impeach Trump, the Republican-controlled Senate would then hold a trial on whether to remove him from office.

Senate Republicans have so far shown little appetite for ousting the president.

The impeachment inquiry met on Thursday for the first time with an adviser to Vice President Mike Pence, but former national security adviser John Bolton failed to heed a request to appear.

Lawmakers are also seeking to find out how much Pence knew about efforts by Trump and those around him to pressure Ukraine to investigate Biden and his son.

Jennifer Williams, a career foreign service officer and special adviser to Pence for Europe and Russia, was testifying to members of the House Foreign Affairs, Intelligence and Oversight committees after receiving a subpoena to compel her testimony.

Williams told investigators she found Trump's July call with Zelensky unusual because it was political, not diplomatic in nature, CNN reported, citing an unnamed source. But she did not raise concerns about the call with her superiors and, when asked what Pence knew, said she never heard him mention anything about investigation of the 2016 elections, Burisma or the Bidens.

Bolton, a foreign policy hawk who was fired by Trump in September, was also called to appear on Thursday but did not show, and his attorney said he would not testify voluntarily.

A House Intelligence Committee official said Bolton has threatened to take the committee to court if it subpoenas him. A congressional source said the inquiry is unlikely to go down that route.

The Washington Post, citing people familiar with Bolton's views, said although he is willing, he wants to see how a court battle between Congress and the White House over the constitutionality of the subpoenas shakes out first.

The battle is likely to go to the Supreme Court and could spill into next year.