US President Donald Trump has defended firing a senior official who testified against him at his impeachment trial, saying he was "very insubordinate".
In a series of tweets, Mr Trump said Lt Col Alexander Vindman, a top Ukraine expert, incorrectly reported the contents of a "perfect" phone call that was at the centre of his impeachment.
He also fired US envoy Gordon Sondland, who testified against him at the trial.
Mr Trump reportedly wants a staff shake-up after his Senate acquittal.
In a historic vote on Wednesday, senators decided not to remove America's 45th president from office on charges arising from his dealings with Ukraine.
On Friday, Lt Col Vindman was escorted from the White House hours before Mr Sondland, then US ambassador to the EU, said he had been advised that the president intended "to recall [him] effective immediately".
Lt Col Vindman's twin brother, Yevgeny Vindman, a senior lawyer for the National Security Council, was also sent back to the Department of the Army on Friday.
In his first official response to the firings, Mr Trump attacked Lt Col Vindman's reputation, as well as coverage by US broadcasters CNN and MSNBC. He intentionally misspelled MSNBC as "MSDNC", a reference to the Democratic National Committee (DNC).
"Actually, I don't know him, never spoke to him, or met him (I don't believe!) but, he was very insubordinate, reported contents of my "perfect" calls incorrectly, &. was given a horrendous report by his superior, the man he reported to, who publicly stated that Vindman had problems with judgement, adhering to the chain of command and leaking information. In other words, "OUT", Mr Trump wrote.
How did Vindman annoy Trump?
Testifying in Congress last November, Lt Col Vindman said he was "concerned" after hearing Mr Trump's "improper" phone call on 25 July last year with Ukrainian President Volodymyr Zelensky.
The call led to Mr Trump's impeachment in December by the House of Representatives for abuse of power and obstruction of Congress. Democratic lawmakers argued that the president had dangled US aid in exchange for political favours.
When asked how he had overcome his fear of retaliation in order to testify, Lt Col Vindman testified: "Congressman, because this is America... and here, right matters."
David Pressman, Lt Col Vindman's counsel, told the BBC: "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.
"The most powerful man in the world - buoyed by the silent, the pliable, and the complicit - has decided to exact revenge."
According to White House sources, Lt Col Vindman had been expecting a transfer, telling colleagues for weeks that he was ready to move back to the defence department, where he still holds active-duty soldier status.
Earlier on Friday, US Defence Secretary Mark Esper said his department welcomes back all of its personnel from assignment. "We protect all of our service members from retribution or anything like that."
On Thursday, Mr Trump mentioned the Vindman twins in a tirade against his political enemies at the White House.
How did Sondland annoy Trump?
Mr Sondland was very clear in his testimony that a White House visit by President Zelensky was conditional on Kyiv launching investigations that could be politically helpful to President Trump.
"Was there a quid pro quo [a favour granted in return for something]?" Mr Sondland asked. "As I testified previously, with regard to the requested White House call and White House meeting, the answer is yes."
Mr Sondland was at that time working with Mr Trump's personal lawyer, Rudy Giuliani, on Ukrainian policy at the explicit direction of the president.
In a statement issued by his lawyer, Mr Sondland said: "I am grateful to President Trump for having given me the opportunity to serve, to Secretary [of State Mike] Pompeo for his consistent support, and to the exceptional and dedicated professionals at the US mission to the European Union.
"I am proud of our accomplishments. Our work here has been the highlight of my career."
Reacting to the firings, Eliot Engel, Democratic chairman of the House Committee on Foreign Affairs, said in a statement: "This is shameful of course. But this is also what we should now expect from an impeached president whose party has decided he is above the law and accountable to no one."
But Republican Congressman Thomas Massie defended firing Lt Col Vindman, saying: "He's a leaker, not a whistleblower... Current Commander in Chief doesn't take orders from a Lt Col!"
Is Mick Mulvaney next?
In comments to media on Friday, Mr Trump said reports that Mick Mulvaney, his acting chief of staff, would be fired were "false", saying he had a "great relationship" with him.
North Carolina lawmaker Mark Meadows is being tipped by the Washington rumour mill as a replacement for Mr Mulvaney.
Mr Meadows, who is retiring from the House of Representatives where he led the hardline conservative Freedom Caucus, travelled with Mr Trump on Air Force One on Friday.
At a rare White House press conference in October, Mr Mulvaney appeared to implicate the president in an alleged corrupt deal with Ukraine by saying: "We do that all the time." Mr Trump was reportedly outraged by the gaffe.
Mr Mulvaney then walked back his comments in a written statement that said: "Let me be clear, there was absolutely no quid pro quo between Ukrainian military aid and any investigation into the 2016 election."
Who else is on the way out?
Former Illinois congressman Joe Walsh, one of the two long-shot challengers to Mr Trump for the Republican presidential nomination, announced on Friday that he is ending his campaign.
Mr Walsh, who received just 1% of the vote in Monday's Iowa caucus, told CNN the modern Republican party "has become a cult".
"I want to stop Trump. I believe he's a threat to this country. He can't be stopped within the Republican Party. Nobody can beat him."
Bill Weld, former governor of Massachusetts, is now the only remaining Republican seeking to oust Mr Trump, who will seek re-election in November.