President Donald Trump listens during a multilateral meeting on Venezuela at the InterContinental New York Barclay hotel during the United Nations General Assembly. File photo: AP Photo/Evan Vucci.

Washington - US President Donald Trump asked Ukrainian President Volodymyr Zelensky to potentially open a corruption investigation into Joe Biden, Trump's political rival, in a phone call in July, a transcript released by the White House showed.

"Biden went around bragging that he stopped the prosecution so if you can look into it ... It sounds horrible to me," Trump is quoted as saying in the transcript. He asked Zelensky to "do us a favour."

The release of the previously confidential transcript comes as Trump's rival Democrats have launched an impeachment inquiry against him, threatening, at the very least, to put immense political pressure on the White House ahead of elections next year.

"There's a lot of talk about Biden's son, that Biden stopped the prosecution and a lot of people want to find out about that so whatever you can do with the Attorney General would be great," the five-page transcript quotes Trump as saying, referring to US Attorney General William Barr.

Zelensky did promise to carry out investigations. "I guarantee as the president of Ukraine that all the investigations will be done openly and candidly," he said.

The transcript itself does not show any pressure by Trump on Zelensky with regards to an alleged direct link between Washington withholding military aid and the new Ukrainian president investigating Biden.

Foreign aid did come up extensively, with Trump saying the US was doing far more than European nations to help Ukraine, and Zelensky agreeing that Washington was doing more than Brussels to enforce sanctions on Russia.

"It turns out that even though logically, the European Union should be our biggest partner but technically the United States is a much bigger partner than the European Union," Zelensky said.

Germany was singled out in particular as talking about Ukraine but not doing enough to counter Russia to achieve the stated objectives of the Western bloc.

After the release, Trump bashed "corrupt reporters" who had "built up" the call, saying the transcript showed there was no pressure.

"The way you had that built up, that call, it was going to be the call from hell and it turned out to be a nothing call," he said.

Trump had insisted there was "no quid pro quo" in the call and denied any wrongdoing, as Biden's Democratic Party has launched an impeachment inquiry into the Republican president.

Fox News quoted a US Justice Department spokesperson as saying there was no request from Trump to Barr to contact Ukraine.

One figure who repeatedly is mentioned in the call is Trump's personal lawyer Rudy Giuliani, who has become a central figure in the saga, though his exact role is unclear, other than that he has admitted to contacting Ukraine in the affair.

Trump met Zelensky at a dinner on the sidelines of the UN General Assembly in New York this week. The two are due to hold a formal bilateral later in the day.

Trump was alleged to have pressured Zelensky during the call to provide information on possible corruption in the 2014 appointment of Biden's son to a senior job at Ukrainian oil and gas company Burisma.

Biden, who is challenging Trump for the presidency, was vice president when his son Hunter joined Burisma's board of directors amid a pivotal strengthening of Ukrainian state ties with the West, particularly the US and the European Union.

Nancy Pelosi, the Speaker of the House of Representatives and the top Democrat who announced the impeachment inquiry on Tuesday, told reporters she had not yet seen the transcript, but repeated her insistance that "Trump will be held accountable ... no one is above the law."

dpa