Joe Biden has called on Donald Trump to release the transcript of a controversial conversation with the leader of Ukraine, as pressure mounted on Democrats to move forward with impeachment.

As Mr Trump said he was not taking seriously the threat of such a censure, and defended his request to Ukraine’s president Volodymyr Zelensky to push ahead with an investigation into Mr Biden and his son, the former vice president urged the White House to come clean.

“I’m calling on the president to release the transcript of the call to let everybody hear what it is,” Mr Biden said while campaigning in Iowa. “Let the House see it, and see what he did.”

Last week reports that a member of the US intelligence services had blown the whistle on a conversation the president had with a world leader sparked a furore.

It quickly emerged the call was to Mr Zelensky, and reports said Mr Trump had asked the newly elected Ukraine leader eight times during their conversation to push ahead with an investigation into claims Mr Biden misused his position as vice president to lobby for the ousting of an allegedly corrupt prosecutor. The prosecutor was said to be looking into an energy company that employed Mr Biden’s son, Hunter.

Ukraine Elections 2019 in pictures Show all 26 1 /26 Ukraine Elections 2019 in pictures Ukraine Elections 2019 in pictures Volodymyr Zelenskiy, centre right, and his wife Olena Zelenska, greet supporters after exit polling gave the comedian a commanding lead AP Ukraine Elections 2019 in pictures Outgoing Ukrainian President Petro Poroshenko raises his hand with his wife Maryna at his party headquarters in Kiev AFP/Getty Images Ukraine Elections 2019 in pictures Ukrainian presidential candidate Volodymyr Zelenskiy reacts following the announcement of the first exit poll REUTERS Ukraine Elections 2019 in pictures Comedian and leading Ukrainian presidential candidate Volodymyr Zelenskiy plays ping pong with a journalist at his election night gathering in Kiev, Ukraine. If no candidate receives more than 50% of the vote, the top two vote-getters will face off in a second-round election on April 21 Getty Ukraine Elections 2019 in pictures Election officials count ballots at a polling station in Kiev during the first round of Ukraine's presidential election, AFP/Getty Ukraine Elections 2019 in pictures People walk in front of St. Sophia Cathedral in Kiev during Ukraine's presidential election AFP/Getty Ukraine Elections 2019 in pictures Ukrainian servicemen sit in a truck to head to a polling station, near the front line with pro-Russian separatists, near Butivka, Donetsk region AFP/Getty Ukraine Elections 2019 in pictures Ukrainian citizens living in Kyrgyzstan vote in Ukraine's presidential election at a polling station in the Ukrainian embassy in Bishke AFP/Getty Ukraine Elections 2019 in pictures Ukrainian citizens living in Kyrgyzstan vote in Ukraine's presidential election at a polling station in the Ukrainian embassy in Bishkek AFP/Getty Ukraine Elections 2019 in pictures Ukrainian citizens living in Kyrgyzstan vote in Ukraine's presidential election at a polling station in the Ukrainian embassy in Bishkek AFP/Getty Ukraine Elections 2019 in pictures Ukrainian citizens living in Kyrgyzstan vote in Ukraine's presidential election at a polling station in the Ukrainian embassy in Bishkek AFP/Getty Ukraine Elections 2019 in pictures A young girl casts the ballot of a man at a polling station in Kiev on the first round of Ukraine's presidential election, on. Exit polls are expected when voting stations close at 8 pm local time (1700 GMT). First preliminary results are expected several hours after. Barring a shock result in which one candidate crosses the 50 percent threshold in the first round, a run-off will be held on April 2 AFP/Getty Ukraine Elections 2019 in pictures Ukrainian comic actor, showman and presidential candidate Volodymyr Zelensky waves in front of voting booths at a polling station during Ukraine's presidential election in Kiev AFP/Getty Ukraine Elections 2019 in pictures Former Ukrainian Prime Minister and presidential candidate Yulia Tymoshenko speaks with the media after casting her ballot at a polling station during Ukraine's presidential election in Kiev AFP/Getty Ukraine Elections 2019 in pictures Former Ukrainian Prime Minister and presidential candidate Yulia Tymoshenko speaks with the media after casting her ballot at a polling station during Ukraine's presidential election in Kiev AFP/Getty Ukraine Elections 2019 in pictures Former Ukrainian Prime Minister and presidential candidate Yulia Tymoshenko speaks with the media after casting her ballot at a polling station during Ukraine's presidential election in Kiev AFP/Getty Ukraine Elections 2019 in pictures Ukrainian comic actor, showman and presidential candidate Volodymyr Zelensky casts his ballot at a polling station during Ukraine's presidential election in Kiev AFP/Getty Ukraine Elections 2019 in pictures Ukrainian comic actor, showman and presidential candidate Volodymyr Zelensky gestures in front of voting booths at a polling station during Ukraine's presidential election in Kiev AFP/Getty Images Ukraine Elections 2019 in pictures Ukrainian comic actor, showman and presidential candidate Volodymyr Zelensky walks with his ballot at a polling station during Ukraine's presidential election in Kiev AFP/Getty Images Ukraine Elections 2019 in pictures A view of the building of the Ukrainian Central Election Commission in Kiev during Ukraine's presidential election. AFP/Getty Ukraine Elections 2019 in pictures Ukrainian President Petro Poroshenko (L) casts his ballot at a polling station in Kiev on the first round of Ukraine's presidential election, The 53-year-old president has positioned himself during the political campaign as the only person able to stand up to the Kremlin and has promised to return Crimea to Ukraine if he is re-elected. AFP/Getty Ukraine Elections 2019 in pictures A voter emerges from the voting booth after filling out her ballot for Ukraine's presidential election in Kiev, Ukraine. If no candidate receives more than 50% of the vote, the top two vote-getters will face off in a second-round election on April 21. Getty Ukraine Elections 2019 in pictures Ukrainian president Petro O. Poroshenko (C), running for re-election, receives his ballot in Ukraine's presidential election on in Kiev, Ukraine. If no candidate receives more than 50% of the vote, the top two vote-getters will face off in a second-round election on April 21 Getty Ukraine Elections 2019 in pictures A woman holds her ballot as she leaves a voting booth at a polling station in Kiev on the first round of Ukraine's presidential election. - Exit polls are expected when voting stations close at 8 pm local time (1700 GMT). First preliminary results are expected several hours after. Barring a shock result in which one candidate crosses the 50 percent threshold in the first round, a run-off will be held on April 21 AFP/Getty Ukraine Elections 2019 in pictures A young girl casts the ballot of a man at a polling station in Kiev on the first round of Ukraine's presidential election. Exit polls are expected when voting stations close at 8 pm local time (1700 GMT). First preliminary results are expected several hours after. Barring a shock result in which one candidate crosses the 50 percent threshold in the first round, a run-off will be held on April 21 AFP/Getty Ukraine Elections 2019 in pictures Ukrainian servicemen queue to cast their ballots at a polling station near the front line line with pro-Russian separatists near Avdiivka, Donetsk region, during the first round of Ukraine's presidential election AFP/Getty

While Mr Biden has previously been cleared of wrongdoing, Mr Trump and his supporters have seized on the episode, seeking to undermine the campaign of the frontrunner among Democrats seeking to challenge him in 2020. On Monday, as he met foreign leaders on the sidelines of the UN General Assembly in New York, Mr Trump insisted he had done nothing wrong. He sought to turn the focus onto Mr Biden.

“Joe Biden and his son are corrupt. If a Republican ever did what Joe Biden did, if a Republican ever said what Joe Biden said, they’d be getting the electric chair right now,” he said.

He added: “I did not make a statement that ‘you have to do this or I’m not going to give you aid’. I wouldn’t do that. I wouldn’t do that.

“There was no pressure put on them whatsoever. I put no pressure on them whatsoever. I could have. I think it probably, possibly would have been OK if I did,” he said.

Mr Trump suggested the White House might release a transcript of the conversation, though he said he worried that it might set a precedent. Later, he wrote on Twitter that Fox News had "bombshell information" that suggested the whistleblower did not have firsthand knowledge of that phone conversation.

Critics of the president claim the episode underscores how Mr Trump considers himself above the law.

They claim the alleged request during a 25 July phone conversation Mr Zelensky was especially brazen as it came just two days after special counsel Robert Mueller had testified to congress about his investigation into Russia’s alleged interference in 2016, and possible collusion with the Trump campaign.

On Capitol Hill, a number of Democrats said the developments crossed a new line. Politico reported that congressman Dean Phillips of Minnesota became the first centrist Democrat to announce support for Mr Trump’s ouster if he had urged the Ukrainian government to investigate his political rival.

Giuliani On Biden/Ukraine

“If the reports are corroborated, we must pursue articles of impeachment and report them to the full House of Representatives for immediate consideration,” Mr Phillips said.

In a letter to her colleagues on Sunday, House speaker Nancy Pelosi wrote: “If the administration persists in blocking this whistleblower from disclosing to congress a serious possible breach of constitutional duties by the president, they will be entering a grave new chapter of lawlessness which will take us into a whole new stage of investigation.”

A number of Democrats, among them Alexandria Ocasio-Cortez, believe Ms Pelosi, 79, is being too timid and have called for the party to move forward with Mr Trump’s impeachment. Ms Pelosi is calculating the move could backfire and hurt Democrats’ fortunes in 2020 as there is little support among the public.

Meanwhile, congressional Democrats demanded documents from the White House about the Mr Trump team’s contacts with Ukraine.

Three House committees – intelligence, foreign affairs and oversight – called on secretary of state Mike Pompeo to produce documents related to contacts between Mr Trump’s lawyer Rudy Giuliani and Ukrainian officials.

The Democratic-led committees said in a letter that they “jointly request documents related to reported efforts by President Trump and his associates to improperly pressure the Ukrainian government to assist the president’s bid for re-election”. They warned they wanted to hear from Mr Pompeo by Thursday to know to whether to obtain subpoenas.

Hunter Biden was hired by the Ukrainian gas company Burisma Holdings in April 2014, two months after Ukraine’s Russia-friendly former president was ousted by protesters, and as the US vice president was heavily involved in US efforts to support the new pro-western government.