U.S. President Donald Trump said on Tuesday the military may look at disciplinary action against former national security aide Alexander Vindman, who testified in Trump's impeachment trial.

Vindman, an army lieutenant colonel who provided some of the most damaging testimony during the House impeachment probe, was ousted from his White House job last week.

Speaking to reporters in the Oval Office, Trump said the military can now deal with Vindman any way they want.

Trump told reporters on Friday he was "not happy" with the 20-year army veteran.

"You think I'm supposed to be happy with him? I'm not," Trump said.

Vindman's status had been uncertain since he testified that he didn't think it was "proper" for Trump to "demand that a foreign government investigate" former vice-president Joe Biden and his son's dealings with the energy company Burisma in Ukraine.

"I privately reported my concerns, in official channels, to the proper authorities in the chain of command. My intent was to raise these concerns because they had significant national security implications for our country," Vindman testified at the impeachment hearing. "I never thought that I'd be sitting here testifying in front of this committee and the American public about my actions."

Gordon Sondland, the U.S. Ambassador to the European Union, said in a statement that Trump had also advised him on Friday that he would be recalled immediately.

Sondland told House impeachment investigators that he worked with Rudy Giuliani, Trump's personal lawyer, on Ukraine at the "express direction" of the president and pushed a "quid pro quo" with Kyiv because it was what Trump wanted.