Joe Biden says Donald Trump must release transcripts from a call where he reportedly pestered Ukraine's leader to investigate his son.

Trump is accused of asking Vladimir Zelensky to probe Hunter Biden at least eight times during a July 25 call, multiple US news outlets reported, citing intelligence sources.

Biden said late Friday that records of the call must be made available "so that the American people can judge for themselves."

Washington's focus is currently glued on a mysterious conversation between Trump and a foreign leader that's reportedly the focus of an August 12 whistleblower complaint from a US intelligence staffer.

On Saturday, Trump repeated allegations made by his lawyer Rudy Giuliani on Thursday that Biden pressured the Ukrainian government to sack a prosecutor investigating his son in 2016.

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Joe Biden says Donald Trump must release transcripts of a call with the president of Ukraine where he reportedly badgered him to probe Biden's son Hunter.

The Democratic Party 2020 candidate issued a statement late Friday demanding the president come clean about the July 25 call with Vladimir Zelensky.

"Donald Trump should immediately release the transcript of the call in question, so that the American people can judge for themselves," he said.

During the call Trump pressed Zelensky to probe Hunter Biden, who works for a Ukrainian natural gas company, at least eight times, the Wall Street Journal reported on Friday.

The Journal also reported that Trump's lawyer, Rudy Giuliani, met top Ukrainian officials including an aide to Zelensky this past summer to discuss an investigation into Biden. Joe Biden, right, and his son Hunter, left, are seen above in April 2016. Teresa Kroeger/Getty Images for World Food Program USA

The Washington Post was first to break the news that Trump's contact with a foreign leader was the subject of a whistleblower complaint filed to the Intelligence Community Inspector General (ICIG) on August 12 by a US intelligence staffer.

Read more: A mysterious exchange between Trump and a foreign leader is Washington's latest obsession. Here's what is actually going on.

Biden also said in the statement on Friday that "there is truly no bottom to President Trump's willingness to abuse his power and abase our country."

Amid widespread criticism the president has defended himself.

Read more: 'DEFCON 1': US officials are rocked by a whistleblower complaint involving Trump's talks with a foreign leader

In a series of tweets on Saturday morning Trump denied the reports, saying Biden had sought to pressure Ukrainian officials into dropping an investigation into Hunter Biden, then working for Burisma Holdings, in 2016.

The allegation first emerged on Thursday after Trump's lawyer Rudy Giuliani said: "I found out this incredible story about Joe Biden, that he bribed the President of the Ukraine in order to fire a prosecutor who was investigating his son."

Trump said of the July 25 phone call on Friday: "I have conversations with many leaders. It's always appropriate. Always appropriate."

"At the highest level always appropriate. And anything I do, I fight for this country."

According to an official statement on Zelensky's website from the time the July 25 call centered around Trump offering his congratulations to Zelensky over his recent election to the presidency.