Impeachment investigators are set to hear from two key witnesses who grew alarmed at how US President Donald Trump and others in his orbit were conducting foreign policy in Ukraine.

David Holmes, a political counsellor at the US Embassy in Kiev, has said he was having lunch with US Ambassador Gordon Sondland this summer when he heard Mr Trump on the phone asking the envoy about the investigations he wanted from the Ukraine president.

The colourful exchange was like nothing he had ever seen, Mr Holmes said in an earlier closed-door deposition.

The second witness – Fiona Hill – has said her National Security Council boss John Bolton cut short a meeting with visiting Ukrainians at the White House when Mr Sondland started asking them about “investigations”.

The two witnesses set to appear on Thursday are the last scheduled for public hearings in an inquiry that has brought hours of testimony from a roster of current and former US government officials defying Mr Trump’s orders not to appear.

David Holmes
David Holmes is set to give evidence to the impeachment inquiry (Jose Luis Magana/AP)

The impeachment inquiry focuses on allegations that Mr Trump sought investigations of former vice-president Joe Biden and his son – and the discredited idea that Ukraine rather than Russia interfered in the 2016 US election – in return for the badly needed military aid and a White House visit the new Ukrainian president wanted to show his backing from the West.

Those testifying publicly this week previously appeared for private depositions, most having received subpoenas compelling them to give evidence.

Mr Holmes has told investigators the call he overheard “was so remarkable that I remember it vividly”.

He said he heard Mr Trump ask: “So he’s going to do the investigation?”

According to Mr Holmes, Mr Sondland replied that Ukrainian President Volodymyr Zelenskiy “will, quote, ‘do anything you ask him to’”.

Ms Hill has said Mr Bolton told her he did not want to be involved in any “drug deal” Mr Sondland and Mr Trump’s acting White House chief of staff Mick Mulvaney were cooking up over the Ukrainian investigations the president wanted.

Mr Sondland, a wealthy hotelier and donor to Mr Trump’s inauguration, appeared before politicians on Wednesday in a marathon session.

Fiona Hill
Former White House adviser on Russia, Fiona Hill, will be another witness on Thursday (Andrew Harnik/AP)

He declared Mr Trump and his lawyer Rudy Giuliani explicitly sought a “quid pro quo” with Ukraine, leveraging an Oval Office visit for political investigations of Democrats. But he also came to believe the trade involved much more.

Mr Sondland testified it was his understanding the president was holding up nearly $400 million in military aid, which Ukraine badly needs with an aggressive Russia on its border, in exchange for the country’s announcement of the investigations.

Mr Sondland conceded Mr Trump never told him directly the security assistance was blocked for the probes, a gap in his account that Republicans and the White House seized on as evidence the president did nothing wrong.

But the ambassador said his dealings with Mr Giuliani, as well as administration officials, left him with the clear understanding of what was at stake.

“Was there a ‘quid pro quo’?” Mr Sondland testified in opening remarks. “With regard to the requested White House call and White House meeting, the answer is yes.”

The rest, he said, was obvious: “Two plus two equals four.”