US ambassador to the European Union Gordon Sondland says he knew that the Trump administration withheld nearly $US400 million in American aid while pressuring Ukraine to investigate President Donald Trump’s political rivals, according to an excerpt of his testimony.

In a reversal of his initial testimony in the impeachment inquiry last month, Mr Sondland said that by the beginning of September, “in the absence of any credible explanation for the suspension of aid, I presumed that the aid suspension had become linked to the proposed anti-corruption statement”.

Mr Sondland said he told a Ukrainian presidential adviser that “resumption of US aid would likely not occur until Ukraine provided the public anti-corruption statement that we had been discussing for many weeks”, in excerpts of his testimony released on Tuesday (local time).

Sondland submitted the supplemental testimony on Monday after depositions by other officials, including US ambassador to Ukraine Bill Taylor, refreshed his memory, the excerpts show.

In testimony on October 17, Sondland did not acknowledge a link between the frozen US aid to Ukraine and the request to investigate one of Mr Trump’s Democratic political rivals.

The House of Representatives’ impeachment investigation is focused on a July 25 phone call in which the Republican Trump asked Ukrainian President Volodymyr Zelenskiy to open an investigation into former US Vice President Joe Biden and his son Hunter Biden.

Hunter Biden was on the board of a Ukrainian gas company that had been investigated for corruption.

Joe Biden is a leading contender for the Democratic Party nomination to run against Trump in the November 2020 election.

Mr Trump froze nearly $US400 million in US military assistance to Ukraine shortly before speaking to Mr Zelenskiy, prompting accusations from Democrats that he had misused taxpayer dollars destined for a vulnerable US ally for personal gain.

Mr Sondland had initially denied knowledge of any link between the aid and Mr Trump’s request that Ukraine investigate the Bidens.

Congressional Democrats also released testimony from Kurt Volker, Mr Trump’s former special representative for Ukraine negotiations.

Witnesses have testified that Mr Volker and Mr Sondland, with Mr Trump’s Secretary of Energy Rick Perry, were known as the “three amigos” responsible for Mr Trump’s unofficial channel to Ukrainian government officials.

Mr Volker resigned as special representative in September. He testified before the House Foreign Affairs, Intelligence and Oversight panels for more than eight hours behind closed doors on October 3.

Mr Sondland, a major Trump donor, testified on October 17.

Mr Perry, a former Texas governor who said he was resigning from his cabinet post as of December 1, has refused to testify so far.

Mr Trump has denied wrongdoing and accused Democrats of unfairly targeting him in hope of reversing his surprise victory in the 2016 presidential election. So far, he has maintained strong support from fellow Republicans in congress.

-AAP