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 Iraq War 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."
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