The former US ambassador to Ukraine on Friday told a House of Representatives impeachment inquiry into President Donald Trump that he ousted her based on "unfounded and false claims" after she had come under attack by his personal lawyer Rudy Giuliani.

Key points: Ms Yovanovitch was told not to attend the hearing by the White House, but was compelled to after Democrats lodged a subpoena for her appearance

Ms Yovanovitch was told not to attend the hearing by the White House, but was compelled to after Democrats lodged a subpoena for her appearance She told Congress Mr Trump had pushed for her sacking even though the department believed she had "done nothing wrong"

She told Congress Mr Trump had pushed for her sacking even though the department believed she had "done nothing wrong" Mr Trump's lawyer Rudy Giuliani suggested she was biased against the President

Marie Yovanovitch, who was abruptly recalled as the US envoy to Ukraine in May, appeared for a closed-door deposition, according to Democratic politicians leading the inquiry, after she had been told by the State Department at the behest of the White House not to show up.

The Democrats said they then issued a subpoena for her appearance and she complied.

Ms Yovanovitch, according to a copy of her opening statement posted online by the Washington Post, said she was told by a senior State Department official about "a concerted campaign against me" and that Mr Trump had pushed for her removal since the middle of last year even though the department believed "I had done nothing wrong".

She also expressed alarm over damage to US diplomacy under Mr Trump and warned about "private interests" circumventing "professional diplomats for their own gain, not the public good".

The impeachment inquiry focuses on a July 25 phone call in which Mr Trump pressed Ukrainian President Volodymyr Zelenskiy to investigate a domestic political rival, Democrat Joe Biden, and Mr Biden's businessman son Hunter Biden.

Mr Giuliani has accused Ms Yovanovitch of blocking efforts to persuade Ukraine to investigate the Bidens.

Mr Giuliani said he provided information to both Mr Trump and the State Department about Ms Yovanovitch, who he suggested was biased against the President.

On Friday, Mr Giuliani said: "I was doing it in my role as a defence lawyer" for Mr Trump.

In her statement, Ms Yovanovitch said she did not know Mr Giuliani's motives for attacking her but that his associates "may well have believed that their personal financial ambitions were stymied by our anti-corruption policy in Ukraine".

Branded by White House as 'bad news'

Democrats have called her removal politically motivated.

"She's a brave woman," Democratic congressman Michael Quigley said of Ms Yovanovitch during a break in the testimony.

Mr Trump discussed Ms Yovanovitch in his call with Mr Zelenskiy, according to a White House summary of the conversation.

"The woman was bad news and the people she was dealing with in the Ukraine were bad news," Mr Trump said.

Mr Zelenskiy agreed with Mr Trump that she was a "bad ambassador" and agreed to investigate the Bidens.

The conversation occurred after Mr Trump had withheld $391 million in congressionally approved security assistance to Ukraine to help it deal with Russia-backed separatists in the eastern part of the country.

"Although I understand that I served at the pleasure of the President, I was nevertheless incredulous that the US government chose to remove an ambassador based, as best as I can tell, on unfounded and false claims by people with clearly questionable motives," Ms Yovanovitch said in her statement.

Mr Giuliani accused Ms Yovanovitch of blocking efforts to persuade Ukraine to investigate Mr Biden and his son. ( AP: Charles Krupa )

Ms Yovanovitch entered the Capitol building for the deposition wearing dark glasses and walked past a crowd of journalists without responding to questions.

A career diplomat who also has served as US ambassador to two other countries, Ms Yovanovitch's stint as envoy in Kiev was cut short when she was recalled to Washington as Trump allies levelled unsubstantiated charges of disloyalty and other allegations against her.

Democrats have accused Mr Trump of pressuring a vulnerable foreign ally to dig up dirt on a domestic rival for his own political benefit.

Mr Biden, the former US vice president, is a leading Democratic contender for the right to face Mr Trump in the November 2020 presidential election.

Mr Trump has denied he did anything wrong on the call.

The investigation of Mr Trump could lead to the approval of articles of impeachment — or formal charges — against the president in the House.

'Defend this great institution'

Ms Yovanovitch warned about Russia's "malign intentions" toward Ukraine and said that if the United States allows Russian actions toward its neighbour to stand "we will set a precedent that the United States will regret for decades to come".

"Today, we see the State Department attacked and hollowed out from within," Ms Yovanovitch said.

"State Department leadership, with Congress, needs to take action now to defend this great institution, and its thousands of loyal and effective employees."

Ms Yovanovitch said the notion she had been disloyal to Mr Trump was "fictitious". ( AP: Efrem Lukatsky )

Ms Yovanovitch said in her statement that the notion she had been disloyal to Mr Trump was "fictitious" and dismissed the idea that former Democratic President Barack Obama's administration had asked her to harm Mr Trump's 2016 presidential campaign.

Ms Yovanovitch was the third key witness to appear in the impeachment inquiry.

On Thursday, two Florida businessmen — Ukraine-born Lev Parnas and Belarus-born Igor Fruman — who had helped Mr Giuliani as he investigated Mr Biden were arrested in what US prosecutors said was a scheme to illegally funnel money to a pro-Trump election committee and other political candidates.

Mr Parnas sought the help of a US congressman to get Mr Trump to remove Ms Yovanovitch, according to the indictment.