A Pentagon official detailed confusion and concern in the US national security system after the White House blocked aid to Ukraine without explanation, according to testimony released by the congressional impeachment panel into US President Donald Trump.

Testimony by Laura Cooper, deputy assistant secretary of defense, is the latest in a series to be made public by the inquiry.

It reveals the internal workings of the administration as Mr Trump insisted that $400 million in aid slated for Ukraine be withheld until the nation's President Volodymyr Zelenskiy publicly launched an investigation into one of Mr Trump's political rivals.

The panel also released testimony from Catherine Croft and Christopher Anderson, both advisers to Ambassador Kurt Volker on Ukraine policy.

The revelations come two days before the impeachment inquiry by the Democratic-led US House of Representatives committees enters a crucial new public phase.

The first on-camera hearings in the investigation are set for later this week and focused on accusations that Mr Trump improperly withheld security money as leverage to pressure Ukraine to investigate political rival Joe Biden.

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 Mr 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 Mr Trump's removal from office.M

Ms Cooper's closed-door testimony on 23 October became the scene of one of the most explosive moments of the inquiry so far, when a group of about two dozen Republican politicians and allies of Mr Trump stormed the secure room where she was speaking.

Critics denounced their actions, saying by taking phones into the secure room, they had compromised US security.

In her testimony, Ms Cooper described at some length the approval process for the $391 million in aid to Ukraine, including that the Pentagon had determined that the Ukrainian government had met anti-corruption requirements for the release of the funds.

Mr Trump and some of his supporters have argued that the funds - approved by the US Congress to help combat Russian-backed separatists in the eastern part of Ukraine - were blocked by Mr Trump to press Mr Zelenskiy's government to fight corruption, not to seek an investigation of Mr 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," Ms Cooper said, according to the transcript.

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

"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."

The investigating 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 Mr Trump over his 25 July call with Mr Zelenskiy.

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

Mr Biden is a leading contender for the Democratic nomination to face the Republican president in the 2020 election. 

His son Hunter Biden served on the board of a Ukrainian energy company called Burisma.

Mr Trump has denied there was a quid pro quo - or exchanging a favour for a favour - in his dealings with Ukraine. 

He defended his call with Mr Zelenskiy as "perfect" and branded the probe a politically motivated hoax. 

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