An associate of Donald Trump's personal lawyer has said the president "knew everything" about their efforts to push the Ukrainian government to investigate Joe Biden's son.
Mr Trump is braced for his impeachment trial to begin on charges that he has dismissed as a "sham", having repeatedly denied that he tried to influence officials.
He has been accused of abusing his power by pressuring President Zelenskiy to launch an investigation into Hunter Biden over his time on the board of Ukrainian oil company Burisma.
Mr Trump's lawyer, Rudy Giuliani, was allegedly directed by the president to encourage Ukraine to look into unsubstantiated corruption accusations against the Bidens.
Joe Biden is one of Mr Trump's likely rivals in this year's US election.
Mr Giuliani told Sky News last year that his client "didn't do a darn thing wrong", but now one of his associates has suggested the president was directly involved.
Lev Parnas, who has been accused of breaking campaign finance laws in his support of Mr Trump's re-election bid, accused him of lying about not knowing who he was and his role in the alleged anti-Biden scheme.
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Asked about "the main inaccuracy" that he could correct over the impeachment inquiry, he told MSNBC: "That the president didn't know what was going on.
"President Trump knew exactly what was going on."
Mr Parnas added: "He was aware of all of my movements. I wouldn't do anything without the consent of Rudy Giuliani or the president. I have no intent, I have no reason to speak to any of these officials."
He insisted that senior government officials in Kiev "were told to meet" with him.
In response to Mr Trump saying he did not know who he was, Mr Parnas said: "He lied. I mean we are not friends, but he knew exactly who we were. I was with Rudy four or five days out of the week."
He described the president's claim as "just ludicrous".
Regarding allegations that Mr Trump threatened to withhold military aid from Ukraine unless they agreed to the Biden investigation, Mr Parnas said: "It wasn't just military aid. It was all aid.
"Basically the relationships would be sour. We would stop giving them any kind of aid."
White House press secretary Stephanie Grisham has dismissed the comments made by Mr Parnas.
She said: "These allegations are being made by a man who is currently out on bail for federal crimes and is desperate to reduce his exposure to prison.
"The facts haven't changed - the president did nothing wrong and this impeachment, which was manufactured and carried out by the Democrats, has been a sham from the start."
The interview on MSNBC was aired on the day that Mr Trump's impeachment reached the trial stage.
Former vice president Mr Biden is among those to have called for the Republican to be removed from office, having described the efforts to have him and his son investigated as an "impeachable act" .
Mr Biden remains the favourite to be nominated as the Democratic candidate in the 2020 presidential election after the latest televised debate between him and his rivals .
Despite the impeachment process reaching the trial stage, Mr Trump is not expected to be removed from office.
The Democrats control the House of Representatives, making it straightforward for proceedings to get this far, but the second chamber of the US government - the Senate - is controlled by the Republicans.
They have 53 seats there compared to 47 for the Democrats, and it would take 51 votes to approve rules or call witnesses during the trial.
If four Republican senators joined with Democrats, they could insist on new evidence.
Mr Trump's trial will be only the third in US presidential history - and the second to be televised.