A White House official who listened in on Donald Trump’s infamous phone call with Ukraine’s president has told an impeachment inquiry that the conversation was “inappropriate” and “improper”.

The president has already attempted to discredit the two latest witnesses to give evidence, people who heard his July phone call, as “Never Trumpers” – a term to describe Republicans who oppose him.

Now it seems the attacks have advanced: On Wednesday morning (Australian time), the White House’s official Twitter account was used to blast the heads of the probe and witnesses.

One White House tweet referred to House Intelligence Committee chair as “shifty” and claimed “fake quotes” were being invented, while another was a tweet of Mr Trump calling the process a “scam” and “kangaroo court”.

Shifty Schiff is inventing fake quotes from the President again. 🤦 pic.twitter.com/KMjmTnFkDn

— The White House 45 Archived (@WhiteHouse45) November 19, 2019

Army Lieutenant Colonel Alexander Vindman told lawmakers that Mr Trump’s insistence on having his main Democratic rival Joe Biden investigated would have had “significant implications” had the public learned of the July 25 call.

But the Iraq war veteran said he felt “it was my duty” to report Mr Trump’s “request – to demand – an investigation into a political opponent, especially (from) a foreign power”.

“There is at best dubious belief that this would be a completely impartial investigation and that this would have significant implications if it became public knowledge,” Mr Vindman told the committee on Wednesday morning (Australian time).

"They're using this impeachment hoax for their own political gain." pic.twitter.com/9sGlHiLpvX

— The White House 45 Archived (@WhiteHouse45) November 19, 2019

It was the third public hearing in the impeachment effort before the US House of Representatives intelligence committee.

Aside from asking Ukraine to launch a probe into Mr Biden, Mr Trump is also being investigated over allegations Ukraine was possibly interference in the 2016 presidential election – a debunked conspiracy theory embraced by some Trump allies.

Mr Vindman has been previously criticised by Mr Trump, along with other witnesses. He told lawmakers that “character attacks” against public servants testifying in the impeachment inquiry were “reprehensible”.

“It is natural to disagree and engage in spirited debate, this has been the custom of our country since the time of our Founding Fathers, but we are better than personal attacks,” he told lawmakers.

Mr Vindman testified alongside Jennifer Williams, an aide to US Vice President Mike Pence, both of whom were among the US officials who listened in on the July 25 call.

#RT @CNN: Jennifer Williams, an aide to VP Mike Pence, responds to Trump's tweet over the weekend claiming she is a "never Trumper" during testimony before the House Intelligence Committee: "It certainly surprised me. I was not expecting to be called out… pic.twitter.com/xLOY3KwrXd

— Ripped Limbo (@LimboRipped) November 19, 2019

Mr Vindman, whose family fled the Soviet Union and settled in the United States, sent out a message to his father from the witness seat.

“Dad, that I am sitting here today, in the US Capitol talking to our elected officials is proof that you made the right decision 40 years ago to leave the Soviet Union and come here to the United States of America in search of a better life for our family. Do not worry, I will be fine for telling the truth,” Vindman testified.

Ms Williams told the committee that Trump’s call with Zelenskiy was unusual because “it involved discussion of what appeared to be a domestic political matter”.

She said the White House budget office had said Trump’s acting chief of staff, Mick Mulvaney, had directed that $US391 million in security aid to Ukraine be put on hold and that she never learned why the assistance was later released in September.

Democrats have accused Mr Trump of using the frozen aid and Zelenskiy’s desire for an Oval Office meeting as leverage to pressure a vulnerable US ally to dig up dirt on political adversaries.

Representative Devin Nunes, the committee’s top Republican and a close Trump ally, did not address the substance of the witness testimony about the president’s actions in his opening statement on Tuesday but unleashed an attack on the media, calling journalists “puppets of the Democratic Party”.

At one point, Schiff interrupted Nunes’ questioning of Vindman that appeared to be aimed at revealing the identity of the intelligence community whistleblower whose complaint about Trump’s July 25 call triggered the impeachment inquiry.

The whistleblower’s identity has remained a secret, but Trump and his allies have repeatedly attacked the individual.

“These proceedings will not be used to out the whistleblower,” Schiff said.

-with AAP