A former national security official told the impeachment inquiry into Donald Trump that US ambassador carried out a controversial "political errand" on behalf of the US president.

Fiona Hill told the House of Representatives investigators she came to realise ambassador Gordon Sondland was not simply operating outside official diplomatic channels, as she and others suspected, but carrying out instructions from Mr Trump.

She said: “He was being involved in a domestic political errand, and we were being involved in national security foreign policy and those two things had just diverged.”

Ms Hill’s comment followed a blistering back-and-forth during questioning from Republicans at the House hearing.

Evidence from Ms Hill and David Holmes, a state department adviser in Kiev, reinforced that Mr Trump used foreign policy for political aims.

Democrats allege Mr Trump was relying on the discredited idea that Ukraine rather than Russia interfered in the 2016 US election as he sought investigations in return for US military aid and a White House visit the new Ukrainian president wanted.

President Donald Trump has denied any wrongdoing Credit: Evan Vucci/AP

Ms Hill and Mr Holmes both told House investigators it was abundantly clear Mr Trump’s lawyer Rudy Giuliani was pursuing political investigations of Democrats and political rival Joe Biden in Ukraine.

Ms Hill said: “He was clearly pushing forward issues and ideas that would, you know, probably come back to haunt us and in fact, I think that’s where we are today.”

And Ms Hill stood up for Lieutenant Colonel Alexander Vindman, the army officer who gave evidence earlier and whom Mr Trump’s allies tried to discredit. He remains on the White House National Security Council.

At one point, Republicans interjected, trying to cut off Ms Hill. The Republican members of Congress had been trying to highlight her differences with Mr Sondland, the ambassador to the European Union who delivered damaging evidence on Wednesday about what he said was Mr Trump’s “quid pro quo” pursuit of the political investigations.

“You may not like the witness’s answer, but we will hear it,” said representative Adam Schiff, the Democratic chairman of the committee.

The Republican congressmen eventually wound down their questions but continued with mini-speeches decrying the impeachment effort. Democrats, in turn, criticised Mr Trump’s actions.

Ms Hill, a former aide to then-national security adviser John Bolton, sternly warned Republican congressmen to stop pushing a “fictional” narrative that Ukraine, rather than Russia, interfered in US elections.

Mr Trump has told others giving evidence in the inquiry that Ukraine tried to “take me down” in the 2016 election. Republicans launched their questioning reviving those theories.

Ms Hill said: “I refuse to be part of an effort to legitimise an alternative narrative that the Ukrainian government is a US adversary, and that Ukraine – not Russia – attacked us in 2016.”

Her evidence also raised fresh questions whether Mr Bolton, who has yet to defy White House orders for officials not to give evidence, would appear in the inquiry. In what was seen as a nudge to her former boss, Ms Hill said those with information have a “moral obligation to provide it”.