WASHINGTON (AFP) - A US ambassador ousted by Donald Trump told a riveting impeachment hearing on Friday (Nov 15) she felt intimidated by the president as he launched an extraordinary, real-time attack midway through her testimony to Congress.

Marie Yovanovitch was recounting to lawmakers how she was abruptly pulled from Ukraine after what she calls a smear campaign, when the president lashed out at the highly-regarded career diplomat, tweeting that "everywhere (she) went turned bad."

Asked on live television what effect Trump's tweet might have on her and other witnesses, Yovanovitch told the panel: "It's very intimidating."

"I can't speak to what the president is trying to do but I think the effect is to be intimidating," she said.

Trump's outburst highlighted the intensity of the public hearings on Capitol Hill, that seek to establish whether the president abused the power of his office for personal political gain in his dealings with Ukraine.

During a break in proceedings, House Intelligence Committee chairman Adam Schiff called it "witness intimidation in real-time by the president."

Schiff would not say whether it was an impeachable offence, but witness intimidation is a crime.

Trump's tweet, in which he declared his "absolute right" to appoint and recall ambassadors, earned a sharp rebuke by a figure at the center of the last US impeachment inquiry.

Ken Starr, the special prosecutor who investigated Democrat Bill Clinton in 1998, said on Fox News that Trump's tweet showed "extraordinarily poor judgment."

The impeachment inquiry in the Democratic-controlled House of Representatives centers on accusations that Trump froze US military aid in an effort to get Ukraine to launch political investigations against his potential 2020 election rival Joe Biden, and his son Hunter.

The investigation threatens to make Trump the third US president to be impeached, after Clinton and before him Andrew Johnson in 1868, although the Senate - where Republicans enjoy a majority - would need to convict him to remove him from office.

'KNEECAPPED'

Prior to her ouster in May, Yovanovitch claims she was targeted by a smear campaign orchestrated in part by Trump's personal attorney Rudy Giuliani - who is accused of helping coordinate the president's effort to pressure Kiev into investigating Biden.

She told lawmakers her inexplicable removal had plunged America's Ukraine policy into "disarray" and damaged Washington's standing on the world stage.

And when she read the memorandum of Trump's July 25 phone call with Ukraine's leader Volodymyr Zelenskiy - in which the US leader disparaged her as "bad news" and said she was "going to go through some things" - she says "it sounded like a threat."

Yovanovitch sharply criticised the Trump administration and expressed alarm that envoys could be so easily "kneecapped" by false accusations and corrupt foreign influences.

"Our Ukraine policy has been thrown into disarray, and shady interests the world over have learned how little it takes to remove an American Ambassador who does not give them what they want," she said.

Yovanovitch levelled severe criticism at the State Department, and by extension Secretary of State Mike Pompeo, charging that "policy process is visibly unravelling."

She expressed grave concerns about the department's failure to push back against forces that "apparently hijacked our Ukraine policy," or to support her in the face of "dangerously wrong" attacks against her.

'CAST ASIDE'

The testimony comes as Democratic lawmakers sharpen their case against the president.

"Ambassador Yovanovitch was serving our nation's interest in fighting corruption in Ukraine," said Schiff, who is overseeing the impeachment probe, in his opening statement.

"But she was considered an obstacle to the furtherance of the president's personal and political agenda. For that, she was smeared and cast aside."

Devin Nunes, the top Republican on the panel, hit back by dismissing what he called "Watergate fantasies."

"The Democrats have convened us once again to advance their operation to topple a duly elected president."

During Wednesday's opening public hearing, senior career State Department official George Kent also spoke of a smear campaign against Yovanovitch, saying he was "alarmed" by it.

Also testifying Wednesday was top envoy to Ukraine William Taylor, who revealed details of a previously unknown call between Trump and another diplomat involved in the irregular Ukraine policy channel.

Taylor said his aide overheard the call between Trump and US Ambassador to the European Union Gordon Sondland, in which Trump asked about the status of the "investigations."

That aide, David Holmes, was set to testify behind closed doors on Friday about the contents of the call - of which Trump says he has no recollection.