A White House official has testified in the impeachment inquiry against US President Donald Trump that a phone call made to ask Ukraine to investigate his political rivals was improper.

Army Lieutenant Colonel Alexander Vindman, the White House National Security Council’s top Ukraine expert, testified at the third public hearing in the impeachment investigation before the US House of Representatives intelligence committee.

He also denounced attacks on witnesses in the investigation.

The inquiry focuses on a 25 July phone call in which Mr Trump asked Ukrainian President Volodymyr Zelenskiy to carry out two investigations that would benefit him politically including one targeting Democratic political rival Joe Biden.

The other involved a debunked conspiracy theory embraced by some Trump allies that Ukraine, not Russia, interfered in the 2016 US election.

“It was inappropriate, it was improper for the president to request – to demand – an investigation into a political opponent, especially (from) a foreign power where 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,” Lt Col Vindman told the committee on Tuesday.

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He 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 25 July call.

Lt. Col. Vindman, an Iraq war veteran who appeared at the hearing wearing his Army uniform and medals, has along with other witnesses been publicly criticised by Mr Trump. 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.

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Donald Trump. Photo: supplied

Mr Trump has criticised both Mr Williams and Lt Col Vindman on Twitter as “Never Trump” witnesses, a term to describe Republicans who oppose him.

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Lt Col 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,” he said.

Ms Williams told the committee that Mr Trump’s call with Mr 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 Mr Trump’s acting chief of staff, Mick Mulvaney, had directed that $US391 million ($NZ608m) in security aid to Ukraine be put on hold and that she never learned why the assistance was later released in September.

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Democrats have accused Mr Trump of using the frozen aid and Mr 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, US House Intelligence Committee Chairman Adam Schiff interrupted Mr Nunes’ questioning of Lt Col Vindman that appeared to be aimed at revealing the identity of the intelligence community whistleblower whose complaint about Mr Trump’s 25 July phone call triggered the impeachment inquiry.

The whistleblower’s identity has remained a secret, but Mr Trump and his allies have repeatedly called for the individual to be identified.

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

– Reuters