Giuliani hires new lawyers for Trump's impeachment probe over Ukraine

President Donald Trump's personal lawyer Rudy Giuliani announced a new legal team to defend his role in asking Ukraine to investigate former Vice President Joe Biden - a mission that places Giuliani at the centre of a House of Representatives effort to impeach Trump.

"I am represented and assisted by Robert Costello and the Pierce Bainbridge firm, in particular, Eric Creizman and Melissa Madrigal," Giuliani said in a series of Twitter messages on Wednesday.

Giuliani also defended his role investigating Ukrainian corruption as work was done as a personal lawyer to protect Trump against "false charges."

"The evidence, when revealed fully, will show that this present farce is as much a frame-up and hoax as Russian collusion, maybe worse, and will prove the President is innocent," Giuliani said of the House impeachment probe. "The investigation I conducted concerning 2016 Ukrainian collusion and corruption was done solely as a defence attorney to defend my client against false charges."

House Democrats charge that Trump withheld $400 million in military aid to Ukraine in pressing the nation to investigate Biden's role in halting an investigation of a Ukrainian energy firm that employed Biden's son, Hunter.

Transcripts of closed-door testimony released by House investigators this week depict several current and former administration officials describing Giuliani as a backchannel running US policy with Ukraine's recently elected President Volodymyr Zelenskyy.

Earlier on Wednesday, the House announced that the first public hearings on the impeachment probe will begin next week.