US President Donald Trump on Friday 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 worked at the National Security Council, was ordered out of the White House.

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 $1 million to Trump's inauguration, said in a brief statement, "I was advised today that the president intends to recall me effective immediately."

The ouster of Vindman, a respected officer who was wounded in Iraq, was even more abrupt, when he was ordered out of his NSC offices at the White House.

He was "escorted out of the White House where he has dutifully served his country and his president," his lawyer David Pressman said in a statement.

"Vindman was asked to leave for telling the truth," Pressman said.

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

- Taking revenge -

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.

On Friday, Trump told reporters that he wants Republicans to retake control of the lower house of Congress in the next election and to "expunge" his impeachment.

When asked earlier Friday whether he wanted Vindman gone, Trump responded with a veiled threat. "I'm not happy with him," he said.

"You think I'm supposed to be happy with him?" Trump said.

Pressman said there was "no question in the mind of any American" why Vindman had been ousted.

"The truth has cost LTC Alexander Vindman his job, his career, and his privacy," he said in a statement. "He served his country, even when doing so was fraught with danger and personal peril."

Pressman said this was why "the most powerful man in the world... decided to exact revenge."

Democratic Senator Ron Wyden echoed this, tweeting that the two firings were "petty retaliation" carried out "for telling the truth."

There was also outrage from Biden, who interrupted a debate with other presidential hopefuls in New Hampshire to encourage the audience onto its feet in honor of Vindman.

- Key testimony -

Vindman served as director of European affairs on the National Security Council, with responsibility for Ukraine.

Crucially, he was present during a July 25 phone call during which Trump asked Ukrainian President Volodymyr Zelensky to open an investigation into Biden.

House Democrats argued that the call was part of a plot to coerce a foreign ally into helping him ruin Biden's chances in November's presidential election.

Subpoenaed by Congress to testify at the House impeachment hearings, the Ukrainian-born Vindman gave damning evidence.

"It is improper for the president of the United States to demand a foreign government investigate a US citizen and a political opponent," Vindman said in testimony that riveted television viewers.

Sondland told lawmakers he followed the president's orders in seeking a "quid pro quo" deal for Ukraine to investigate Biden in exchange for getting Zelensky a coveted White House visit.

Sondland said Trump's personal lawyer Rudy Giuliani led the effort at Trump's direction to pressure Zelensky 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.