A senior US diplomat has revised his evidence to investigators considering an impeachment inquiry against President Donald Trump.

Gordon Sondland, the US ambassador to the European Union, admitted he knew about the administration's alleged quid pro quo with Ukraine.

He had told a congressional committee leading the impeachment inquiry into Mr Trump that he did not know about a scheme to use US foreign policy to advance the president's political interests.

But Mr Sondland has changed his stance after saying he has been refreshed by statements from other key witnesses.

In evidence released on Tuesday, he admitted that he remembers telling a top aide to Ukraine President Volodymyr Zelenskiy that military aid to the country would probably not resume until Ukraine had provided a public anti-corruption statement, as Mr Trump wanted.

Mr Sondland said that, by the beginning of September, he "presumed that the aid suspension had become linked to the proposed anti-corruption statement".

US President Donald Trump waves to members of the media prior to boarding Marine One on the South Lawn of the White House in Wahinsgton DC | Image: Joyce Boghosian/Zuma Press/PA Images

He also said that soon after, he "came to understand" the statement would have to come from Mr Zelenskiy himself.

Mr Trump has said he did nothing wrong when he called Mr Zelenskiy in July, sparking the scandal, and only asked for a favour.

That favour was to announce an investigation into unfounded corruption allegations involving Hunter Biden, son of Democrat presidential candidate Joe Biden, seen as Mr Trump's biggest potential rival in 2020.

The new evidence came as a shock to Washington because Mr Sondland is a loyal Trump administration official, who donated US$1m to his inauguration.

The next Trump official committee members want to hear from is Mick Mulvaney, Mr Trump's acting chief of staff, who admitted publicly to the quid pro quo before retracting his comments.

The White House has instructed its officials not to comply with the impeachment inquiry being led by House Democrats, and it is uncertain if Mr Mulvaney will appear.

Main image: Gordon Sondland, US ambassador to the European Union, arrives at the Capitol Building in Washington DC for his deposition as part of the house's impeachment inquiry | Image: CQ-Roll Call/SIPA USA/PA Images