An aide to the US President's personal lawyer claims Donald Trump "knew exactly what was going on" with the efforts to pressure Ukraine into investigating former vice-president Joe Biden and his son.

Key points: Lev Parnas said the request to Ukraine to investigate Joe Biden and his son Hunter was never about potential corruption

Lev Parnas said the request to Ukraine to investigate Joe Biden and his son Hunter was never about potential corruption Mr Parnas, who is facing separate criminal charges, said he delivered an ultimatum to Ukraine that aid money would be withheld if Mr Biden was not investigated

Mr Parnas, who is facing separate criminal charges, said he delivered an ultimatum to Ukraine that aid money would be withheld if Mr Biden was not investigated Rudy Giuliani said his statements were "sad" but did not deny them, saying "I'm not responding yet"

Lev Parnas, a close associate of Mr Trump's lawyer, Rudy Giuliani, made several potentially explosive claims in a televised interview with US TV network MSNBC.

Mr Parnas, who is facing separate criminal charges, said Mr Giuliani was never investigating corruption and that the intention was to damage Mr Biden — who was viewed as a potential threat to Mr Trump's 2020 re-election chances.

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

The day after Mr Parnas said he delivered the message, the US State Department announced that Vice-President Mike Pence would no longer be attending the inauguration of Ukrainian President Volodymyr Zelenskiy.

Mr Parnas alleged Mr Trump ordered Mr Pence to avoid the inauguration — at the behest of Mr Giuliani — to send a clear message to the incoming Ukrainian administration that they needed to take seriously the demand for an investigation into Mr Biden.

Donald Trump Jr, Tommy Hicks Jr, Lev Parnas and Igor Fruman in May 2018. ( AP: The Campaign Legal Center )

'President Trump knew exactly what was going on'

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

"President Trump knew exactly what was going on," said Mr Parnas, a Soviet-born Florida businessman facing a raft of 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."

If true, Mr Parnas's account undercuts a key Republican defence of Trump deployed during the ongoing impeachment fight — that Mr Trump's withholding of vital military aid to Ukraine last summer was not a quid pro quo for an investigation into Mr Biden, because Mr Zelenskiy didn't know the aid money was being withheld.

Mr Giuliani called Mr Parnas's statements "sad".

"I feel sorry for him," Mr Giuliani said. "I thought he was an honourable man. I was wrong."

Asked directly if Mr Parnas was lying, he replied, "I'm not responding yet".

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

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

The Justice Department said in September that Mr Trump had not spoken to Attorney-General Barr about having Ukraine investigate the Bidens and that the Attorney-General had not discussed Ukraine with Mr Giuliani.

Justice Department spokeswoman Kerri Kupec said Wednesday that Mr Parnas's claims were "100 per cent false."

Democrats release swathe of documents

The new accusations came as House Democrats made public a trove of documents, text messages and photos from Mr Parnas's smartphones that appeared to verify parts of his account.

A federal judge earlier this month ruled that Mr Parnas could provide the materials to Congress as part of impeachment proceedings into Mr Trump.

The documents could add pressure on the Senate as it debates whether to hear witnesses in the impeachment trial.

The text and phone records show Mr Parnas communicating with Mr Giuliani multiple times a day before the removal of former Ambassador to Ukraine Marie Yovanovitch, as well as a handwritten note that mentions asking Ukraine's president to investigate "the Biden case".

The Associated Press reported in October that Mr Zelenskiy had sought a meeting on May 7 with a small group of key advisers to seek advice about how to navigate the insistence from President Trump and Mr Giuliani for a probe into the Bidens.

This Facebook screenshot shows President Donald Trump standing with Lev Parnas at the White House in Washington, posted on May 1, 2018. ( The Campaign Legal Center via AP )

He expressed his unease about becoming entangled in the American elections, according to three people familiar with the details of the three-hour meeting.

They spoke on the condition of anonymity because of the diplomatic sensitivity of the issue, which has roiled US-Ukrainian relations.

One of the documents released by Democrats is a handwritten note on stationery from the Ritz-Carlton Hotel in Vienna that says "get Zalensky [sic] to Annouce [sic] that the Biden case will be Investigated."

Ms Parnas told MSNBC's Rachel Maddow he took the notes as he was speaking by phone to Mr Giuliani, receiving precise instructions about the demands Mr Trump wanted to convey to Mr Zelenskiy's team.

Mr Trump asked Mr Zelenskiy in a July 25 call to investigate the Bidens.

Hunter Biden served on the board of a gas company based in Ukraine.

The documents were sent to the House Judiciary Committee by three other House panels "to be included as part of the official record that will be transmitted to the Senate along with the Articles of Impeachment," according to a statement.

Some of the materials were made public, while others were blacked out and marked as sensitive.

Mr Parnas pleads not guilty to charges

Mr Parnas and his business partner, Igor Fruman, both US citizens who emigrated from the former Soviet Union, were indicted last year on charges of conspiracy, making false statements and falsification of records.

Prosecutors allege they made outsize campaign donations to Republican causes after receiving millions of dollars originating from Russia. The men have pleaded not guilty.

Mr Trump has repeatedly denied knowing Mr Parnas and Mr Fruman, despite numerous photos that have emerged of the men together.

Among the materials released from Mr Parnas's phone this week were more photos of him with Trump, as well as the President's son Donald Trump Jr, first daughter Ivanka Trump and her husband, Jared Kushner.

Asked by Maddow about Mr Trump's denials of knowing him, Mr Parnas said he had spoken one-on-one with the President numerous times.

"He lied," Mr Parnas said of the President.

"I mean, we're not friends. Me and him didn't watch football games together, we didn't eat hot dogs. But he knew exactly who we were, who I was especially."

ABC/AP