President Donald Trump repeatedly urged Ukraine's president during a telephone call in July to investigate former Vice President Joe Biden's son Hunter and his involvement with a Ukraine natural gas company, a new report says.

The Wall Street Journal reported that Trump encouraged Ukraine President Volodymyr Zelensky "about eight times to work with Rudy Giuliani, his personal lawyer, on a probe, according to people familiar with the matter."

Biden called on President Donald Trump Friday to release the transcript of a call that is the subject of a whistleblower complaint so "the American people can judge for themselves."

"If these reports are true, then there is truly no bottom to President Trump's willingness to abuse his power and abase our country. This behavior is particularly abhorrent because it exploits the foreign policy of our country and undermines our national security for political purposes," Biden said in a statement. "It means that he used the power and resources of the United States to pressure a sovereign nation — a partner that is still under direct assault from Russia — pushing Ukraine to subvert the rule of law in the express hope of extracting a political favor. Such clear-cut corruption damages and diminishes our institutions of government by making them tools of a personal political vendetta. At minimum, Donald Trump should immediately release the transcript of the call in question, so that the American people can judge for themselves, and direct the Office of the Director of National Intelligence to stop stonewalling and release the whistleblower notification to the Congress."

Biden is the current front-runner in the race to win the Democratic presidential nomination and face the Republican nominee, expected to be Trump, in the 2020 election.

The White House did not immediately respond to a request for comment from CNBC about the Journal report.

The Journal report came amid a widening controversy in Washington over another report, by The Washington Post, that Trump had been the subject of a complaint by a whistleblower related to a purported promise he had made related to Ukraine in a conversation with a foreign leader.

The Post did not identify who the leader was, but it noted that the whistleblower complained two and a half weeks after Trump spoke with the newly elected Ukraine president, Zelensky.

Meanwhile, Trump's lawyer Giuliani said in a television interview that he had asked Ukraine officials to investigate Joe Biden.

The Journal's new report came as a top Ukraine official reportedly said that Trump "is looking" for Ukraine officials to investigate business dealings of Biden's son in that country in an effort "to discredit" Biden as he seeks the Democratic presidential nomination.

The official, Anton Geraschenko, told The Daily Beast that Ukraine is ready to investigate Hunter Biden's relationship with the Ukraine gas company "as soon as there is an official request."

But, he added, "Currently there is no open investigation."

Geraschenko is a senior advisor to Ukraine's interior minister, who would be in charge of any investigation of Hunter Biden.

"Clearly, Trump is now looking for kompromat to discredit his opponent Biden, to take revenge for his friend Paul Manafort, who is serving seven years in prison," Geraschenko told The Daily Beast.

Kompromat is a Russian word that means "compromising material."

Manafort served for several months in 2016 as the Republican Trump's presidential campaign manager and is now in federal prison for multiple crimes related to his consulting work for a pro-Russian political party in Ukraine.

His tenure with the Trump campaign ended in August 2016 after The New York Times reported that $12.7 million in secret payments earmarked for him were recorded in ledgers of that party, the Ukraine Party of Regions.

Manafort eventually was prosecuted by special counsel Robert Mueller as part of Mueller's wide-ranging probe into Russian interference in the election that year.

Giuliani for months has been pushing investigations in Ukraine related to two different issues, and has continued to do so as questions about the whistleblower has roiled Congress.

Giuliani's pressure in turn led House Democrats earlier in September to ask Trump's White House counsel for all records relating to efforts "to coerce the Ukrainian government into pursuing two politically motivated investigations under the guise of anti-corruption activity."

"The first is a prosecution of Ukrainians who provided key evidence against Mr. Trump's convicted campaign manager Paul Manafort," three top House Democrats wrote to White House Counsel Pat Cipollone. "That investigation aims to undercut the Mueller Report's overwhelming evidence that Russia interfered in the 2016 election to support Trump's campaign."

"The other case targets the son of [Joe Biden], who is challenging Mr. Trump for the presidency in 2020."