US President Donald Trump "knew exactly what was going on" in an alleged scheme to pressure Ukrainian officials into investigating political rival Joe Biden, a close associate of Trump's personal lawyer has said.

Lev Parnas, an associate of Rudy Giuliani, made several potentially explosive claims in a televised interview on Wednesday night with US broadcaster MSNBC.

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"President Trump knew exactly what was going on," said Parnas, a Soviet-born Florida businessman facing number criminal charges related to campaign finance violations.

"He was aware of all my movements. I wouldn't do anything without the consent of Rudy Giuliani, or the president."

Parnas said he delivered an ultimatum in May to the incoming president of Ukraine that no senior US officials would attend his inauguration and all US aid to the war-torn country would be withheld if an investigation into Biden was not announced.

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The day after Parnas said he delivered the message, the US State Department announced that Vice President Mike Pence would no longer be attending Volodymyr Zelenskyy's inauguration.

Parnas alleged that Trump ordered Pence to stay away, at the behest of Giuliani, to send a clear message to the incoming Ukrainian administration that they need to take seriously the demand for an investigation into Biden, a Democratic presidential candidate seen as a potential threat to Trump's 2020 reelection.

Parnas said every communication he had with Zelenskyy's team was at the direction of Giuliani, whom he regularly overheard briefing Trump about their progress by phone.

Allegations against Barr

If true, Parnas's account undercuts a key Republican defence of Trump deployed during the ongoing impeachment fight - that Trump's withholding of vital military aid to Ukraine last summer was not a quid pro quo for Biden investigations because Zelenskyy did not know the money was being held up.

Giuliani called Parnas's statement "sad".

"I feel sorry for him," Giuliani said in a text to a reporter from The Associated Press. "I thought he was an honourable man. I was wrong."

Asked directly if Parnas was lying, Trump's lawyer replied, "I'm not responding yet."

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In the interview, Parnas also said he had heard Giuliani and another Trump-aligned defence lawyer, Victoria Toensing, briefing Attorney General William Barr by phone about their efforts to pressure the Ukrainian government into announcing the investigation in Biden and his son Hunter's business dealings.

"Barr was basically on the team," Parnas said.

The US Justice Department said in September that Trump had not spoken to Barr about having Ukraine investigate the Bidens and that the attorney general had not discussed Ukraine with Giuliani. Justice Department spokeswoman Kerri Kupec said on Wednesday that Parnas's claims were "100 percent false".

The interview came just hours after Democrats released a trove of documents obtained from Parnas, including a handwritten note that mentions asking Zelenskyy to investigate the Bidens.

The documents, obtained as part of the impeachment investigation, show Parnas communicating with Giuliani and Toensing before the removal of former US Ambassador to Ukraine Marie Yovanovitch.