[WASHINGTON] US lawmakers heard on Tuesday from two direct witnesses to the Ukraine call at the heart of the impeachment inquiry of Donald Trump, both of whom said in open session for the first time they were surprised and concerned by the president's demands for investigations of Joe Biden.

One of the witnesses, Lieutenant Colonel Alexander Vindman, said he was so alarmed by what he heard on the call that he reported the "inappropriate" discussion to lawyers of the National Security Council "out of a sense of duty."

The other witness, Jennifer Williams, a foreign service advisor to Vice-President Mike Pence, said Mr Trump's reference to Mr Biden in the July 25 call with Ukraine's Volodymyr Zelensky was "unusual" in that it delved into domestic US politics.

Ms Williams and Mr Vindman, a senior NSC official, were among four key witnesses testifying on the third day of impeachment hearings into whether Mr Trump abused the power of his office.

Democrats are seeking to show that Mr Trump leveraged nearly US$400 million in military aid and a White House meeting with Mr Zelensky to extract a commitment from the new Ukrainian leader to probe former vice-president Biden and son Hunter, who served on the board of a Ukraine energy firm.

"It is improper for the president of the United States to demand a foreign government investigate a US citizen and a political opponent," Mr Vindman told the House Intelligence Committee hearing.

"This would have significant implications if it became public knowledge and it would be perceived as a partisan play."

The Trump-Zelensky call has become the fulcrum of the investigation.

Ms Williams, a career foreign service officer detailed to Mr Pence's office, also listened to the call.

"I found the July 25th phone call unusual because ... it involved discussion of what appeared to be a domestic political matter," she told the hearing.

"The reference to Biden sounded political to me."

The open testimony amplifies the depositions that both officials delivered during the closed-door portion of the inquiry.

Republicans had savaged the secrecy of the closed-door sessions, and have also sought to discredit several of the witnesses who have defied White House orders not to speak with investigators.

On Monday, in a letter to Republican investigators, Senator Ron Johnson made the unsubstantiated accusation that "it is entirely possible" that Mr Vindman has never accepted Mr Trump as a legitimate president.

'REPREHENSIBLE'

Last week during testimony by the former ambassador to Ukraine, Mr Trump launched an extraordinary attack against her via Twitter. He also criticised Ms Williams after her deposition when she described Mr Trump's effort to pressure Ukraine as "inappropriate."

Mr Vindman took issue with the attacks in his opening statement.

"The character attacks on these servants are reprehensible," Mr Vindman said. "We are better than personal attacks."

Mr Vindman was born in Ukraine and moved to America as a child, and he movingly addressed his father during his testimony, saying he "made the right decision 40 years ago to leave the Soviet Union" and seek a better life for his family in the United States.

"Do not worry, I will be fine for telling the truth."

Mr Trump faces a week of potentially damning testimony in the Ukraine scandal.

But in a surprising move, Mr Trump, faced with the prospect of becoming only the third president in US history to be impeached, said on Monday he might himself testify in the probe.

Nine officials in total are set to testify in public hearings in the House of Representatives this week.

They include Gordon Sondland, the US ambassador to the European Union who allegedly transmitted to the Ukraine government Mr Trump's demands for help in finding dirt on his Democratic rivals ahead of next year's presidential election.

The hearings also include diplomats who previously testified privately that Mr Trump and Mr Sondland repeatedly pushed Kiev to open investigations into the Bidens and withheld nearly military aid and a White House meeting as pressure.

Later Tuesday, Kurt Volker, the former special US envoy to Ukraine, and National Security Council official Timothy Morrison will testify.

The House investigation could conceivably wrap up this week, with evidence prepared to be sent to the House Judiciary Committee to draw up articles of impeachment.

Democrats have demanded testimony from several senior administration officials, notably White House chief of staff Mick Mulvaney, who personally discussed Ukraine policy with Mr Trump, and have also subpoenaed internal records.

The White House, however, has refused to let them testify and release the records.

Mr Trump's impeachment by the Democratic-controlled House, which appears increasingly likely, would place Mr Trump on trial in the Senate, where a Republican majority could protect him from removal.

AFP