US President Donald Trump has fired two of the highest-profile witnesses in his impeachment probe, sparking accusations that he is on a campaign of revenge.

Trump recalled his ambassador to the European Union, Gordon Sondland, just hours after Lieutenant Colonel Alexander Vindman, a decorated soldier who was the top Ukraine expert at the National Security Council (NSC), was ordered out of the White House on Friday.

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Vindman's twin brother Yevgeny, also a lieutenant colonel who worked as a lawyer in the NSC, was fired simultaneously, US media reported.

The firings came two days after the Republican-majority Senate acquitted Trump of charges that he abused his office and one day after he gave a victory speech branding his opponents as "evil".

Sondland, a political appointee who got his post after donating $1m to Trump's inauguration, said in a brief statement: "I was advised today that the president intends to recall me effective immediately."

The removal Vindman, a respected officer who was wounded in Iraq, was even more abrupt.

Vindman was ordered out of his NSC offices and escorted out of the White House, lawyer David Pressman said in a statement, adding that the move was retribution for Vindman's testimony.

"There is no question in the mind of any American why this man's job is over, why this country now has one less soldier serving it at the White House. LTC Vindman was asked to leave for telling the truth," Pressman said.

Trump has described the impeachment process as a hoax, denying there was anything wrong in his push for Ukraine to open a politically embarrassing investigation into Democratic presidential candidate Joe Biden's family.

Petty retaliation

Vindman testified to the US House of Representatives impeachment inquiry in November that Trump made an improper demand of Ukrainian President Volodymyr Zelenskyy in a July phone call that became the centrepiece of the probe into the Republican president.

Vindman told a Democratic-run committee "I couldn't believe what I was hearing" in the phone call. Trump asked Zelenskyy to launch investigations into both Democratic rival Joe Biden and a widely debunked conspiracy theory that Ukraine, not Russia, was behind meddling in the 2016 US presidential election.

In that appearance, Vindman also downplayed concerns that he would suffer payback for speaking out. "I will be fine for telling the truth," he said.

A spokesman for the NSC declined to comment.

Trump emerged victorious from his trial this week with a vote in the Senate, controlled by fellow Republicans who rejected abuse of power and obstruction of justice charges against him.

Asked earlier on Friday about media reports that he might remove Vindman, Trump told reporters: "I'm not happy with him. You think I'm supposed to be happy with him? ... They're going to be making that decision."

A source familiar with the situation told Reuters news agency that Vindman would be reassigned to the US Department of Defense.

Vindman's two-year stint at the White House was due to end in July.

Sondland, a political appointee who got his post after donating $1m to Trump's inauguration, testified that Trump had sought a 'quid pro quo' deal with Ukraine to investigate former Vice President Biden and his son [File: Andrew Harnik/AP]

For his part, Sondland told legislators he followed the president's orders in seeking a "quid pro quo" deal for Ukraine to investigate Biden in exchange for getting Zelenskyy a coveted White House visit.

Sondland said Trump's personal lawyer Rudy Giuliani led the effort, at Trump's direction, to pressure Zelenskyy for the investigation and that top officials in the White House and State Department knew about it.

That testimony helped build the case leading to Trump becoming only the third president ever impeached by Congress, before his acquittal this week.

Another senior White House aide who testified during impeachment proceedings, Jennifer Williams, left this week for a post at the US military's Central Command, according to Bloomberg News.

Trump has cast both Vindman and Williams as "Never Trumpers" who oppose him.

In a social media post, Democratic Senator Ron Wyden said the firings were "petty retaliation" carried out "for telling the truth."