Washington - US President Donald Trump lashed out at a government official who will testify in public in the coming week, as part of the impeachment inquiry, a move that comes on the heels of a previous barb that Democrats said was witness intimidation.

Jennifer Williams testified in a closed-door session that she was concerned the July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky amounted to an "unusual and inappropriate" demand that Ukraine probe Joe Biden, his domestic rival.

Trump tweeted that Williams, "whoever that is" should meet with other opponents of the president and "work out a better presidential attack." Williams works for the vice president and will testify on Tuesday.

Marie Yovanovitch, the ousted ambassador to Ukraine - who seems to have been removed to enable a channel to pressure on Zelensky, according to other testimonies - was testifying before Congress on Friday when Trump bashed her career on Twitter.

Trump had previously disparaged Yovanovitch in his call with Zelensky, calling her "bad news" and cryptically saying: "She's going to go through some things."