A White House official has testified in the impeachment inquiry against US President Donald Trump that a phone call the president made to try to get Ukraine to investigate his political rivals was improper, and he denounced attacks on witnesses in the investigation.

Army Lieutenant Colonel Alexander Vindman, the White House National Security Council's top Ukraine expert, testified at the third public hearing in the impeachment effort before the US House of Representatives Intelligence Committee.

The inquiry focuses on a 25 July phone call in which Mr Trump asked Ukrainian President Volodymyr Zelensky 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.

Lt Col Vindman was 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 been publicly criticised by Mr Trump along with other witnesses. 

He said that "character attacks" against public servants testifying in the impeachment inquiry were "reprehensible".

Donald Trump

"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," Lt Col Vindman said.

In his prepared opening statement to the committee, he had referred to "vile character attacks" and said "we are better than callow and cowardly attacks," but did not use those words at the hearing.

Mr Trump has attacked both Jennifer Williams, an adviser to Vice President Mike Pence, and Lt Col Vindman on Twitter as "Never Trump" witnesses, a term to describe Republicans who oppose him.

Ms Williams told the committee that Mr Trump's call with Mr Zelensky was unusual because it "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 $391m in security aid to Ukraine be put on hold and that she never learned why the assistance was later released in September.

Ms Williams testified behind closed doors this month that some of Mr Trump's comments in the July 25 call were "inappropriate".

US Ambassador to the EU Gordon Sondland is due to testify in the impeachment inquiry

Democrats have accused Mr Trump of using the frozen aid and Mr Zelensky's desire for an Oval Office meeting as leverage to pressure a vulnerable US ally to dig up dirt on political adversaries. Mr Trump is seeking re-election next year.

Mr Trump, who faces becoming only the third president in US history to be impeached, tweeted early yesterday that he is "strongly" considering testifying to defend himself against allegations that he abused his powers in seeking foreign help for the 2020 election.

He tweeted that House leader Nancy Pelosi suggested "that I testify about the phony Impeachment Witch Hunt". 

"She also said I could do it in writing," he said.

"Even though I did nothing wrong, and don't like giving credibility to this No Due Process Hoax, I like the idea & will, in order to get Congress focused again, strongly consider it!"