The release on Monday of the testimony given by Laura Cooper, deputy assistant secretary of defence, in an earlier closed-door session came two days before the impeachment inquiry enters a crucial new phase.

The first public hearings in the investigation - focused on Trump's request that Ukraine investigate political rival Joe Biden and whether he withheld security aid as leverage - are set for Wednesday and Friday.

Cooper described at some length the approval process for the $US391 million ($570 million) in aid to Ukraine, including that the Pentagon had determined that Kiev had met anti-corruption requirements for the release of the funds.

Trump and some of his supporters have argued that the funds - approved by the US Congress to help combat Russia-backed separatists in the eastern part of Ukraine - were blocked by Trump to press the Zelenskiy government to fight corruption, not to seek an investigation of Biden and his son.

"All of the senior leaders of the US national security departments and agencies were all unified in their - in their view that this assistance was essential," Cooper said, according to the transcript.

Cooper said the US military was concerned that denying US assistance would make it harder for Ukraine to negotiate with Moscow.

"They are trying to negotiate a peace with Russia, and if they are seen as weak, and if they are seen to lack the backing of the United States for their armed forces, it makes it much more difficult for them to negotiate a peace on terms that are good for Ukraine."

On Wednesday and Friday, US diplomats William Taylor, George Kent and Marie Yovanovitch are due to detail in public their concerns, previously expressed in testimony behind closed doors, that Trump and his administration sought to tie the security aid to investigations that might benefit his 2020 re-election bid.

The public testimony before the House Intelligence Committee will be carried by major broadcast and cable television networks and is expected to be viewed by millions of people, as Democrats seek to make the case for Trump's potential removal from office.

The panel's Democratic chairman, Representative Adam Schiff, has been a target of the Republican president's attacks since the impeachment probe was launched in September after a whistleblower within the US intelligence community brought a complaint against Trump over his July 25 call with Ukrainian President Volodymyr Zelenskiy.

Democrats, who control the House, have argued that Trump abused his power in pressing a vulnerable US ally to carry out investigations that would benefit Trump politically.

Biden is a leading contender for the Democratic nomination to face the Republican president in the 2020 election. Hunter Biden served on the board of a Ukrainian energy company called Burisma.

Trump has denied there was a quid pro quo - or exchanging a favour for a favour - in his dealings with Ukraine, defended his call with Zelenskiy as "perfect" and branded the probe a politically motivated "hoax".

Trump wrote on Twitter on Monday that the inquiry should be ended and the unnamed whistleblower, the whistleblower's lawyer and "Corrupt politician" Schiff should be investigated for fraud.

Democrats consider the open hearings to be crucial to building public support for a vote on articles of impeachment - formal charges - against Trump.

If that occurs, the 100-seat Republican-controlled Senate would hold a trial on the charges. Republicans have so far shown little support for removing Trump from office, which would require two-thirds of senators present to vote to convict him.

Australian Associated Press