The president, speaking to the media ahead of a cabinet meeting, was asked about Lt Col Alexander Vindman, a national security council expert who was testifying to the House impeachment hearings on the 25 July phone call between Mr Trump and his Ukrainian counterpart which sparked the probe.
Lt Col Vindman was listening to the call, in which Mr Trump appeared to pressure Volodymyr Zelenksy into starting investigations into his political opponents in return for releasing US military aid.
The Iraq War veteran was wearing his military uniform at the hearing and at one point insisted on being addressed as Lt Col Vindman after Republican congressman Devin Nunes called him Mr Vindman.
Mr Trump, appearing in public for the first time since an unscheduled medical visit at the weekend, mispronounced the military veteran’s name and mocked him for wearing his uniform.
He said: “I don’t know him. I don’t know, as he says, the ‘lieutenant colonel’. I understand that somebody had the misfortune of calling him ‘Mr’ and he corrected them.
“I never saw the man. I understand now he wears his uniform when he goes in. No, I don’t know Vind-e-man at all.”
He added: “What I do know is that even he said the transcript was correct, and if anybody reads the transcripts – I had two calls with the president of Ukraine who, by the way said there was no pressure whatsoever, there was no anything.”
Mr Trump has repeatedly described the 25 July phone call as “perfect” and urged people to read what he calls the “transcript”, which is actually a reconstructed memo.
However, while he says it clears him of any wrongdoing, the Democrats see it as one of their key pieces of evidence. Lt Col Vindman himself testified that he was concerned enough by the phone call that he contacted an intelligence official to share his worries.
Earlier, he had hit out at what he called “vile” attacks on his colleagues, apparently directing his comments at Mr Trump and his allies, who have tried to smear diplomats giving evidence at the hearings.
He said: “I want to state that the vile character attacks on these distinguished and honourable public servants is reprehensible.”
A military spokesman told ABC News: "A soldier performing duties in an official capacity will normally be in uniform."
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