(Photo: AFP)

A White House transcript released Wednesday confirmed that President Donald Trump asked his Ukrainian counterpart to probe political rival Joe Biden -- a day after Democrats seized on the explosive allegation to launch an impeachment process.

The official memorandum of a July 25 phone call with President Volodymyr Zelensky shows Trump saying US Attorney General Bill Barr and the president's personal lawyer Rudy Giuliani would be in touch about probing Biden and his son's activities related to Ukraine.

"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," Trump said.

Democrats, who opened formal impeachment proceedings against the president on Tuesday, are investigating whether he pressured a foreign government to look into a political opponent, and if he used a delayed $400 million aid package as leverage.

As Barack Obama's vice president, Biden and other Western leaders pressured Ukraine to get rid of the country's top prosecutor, Viktor Shokin, because he was seen as not tough enough on corruption.

"Biden went around bragging that he stopped the prosecution so if you can look into it... It sounds horrible to me," Trump told Zelensky in the call.

The transcript, which is a summary rather than a word-for-word record, did not show Trump explicitly tying aid to Ukraine to Zelensky probing Biden, which has fed the calls to impeach the US leader.

But the two did discuss aid, which had been frozen by Trump at the time and was only released in recent weeks.

The memo also shows Trump asking Zelensky for a "favor" on an issue unconnected to Biden -- just after noting that the US had been "very, very good to Ukraine."

Democratic leader Nancy Pelosi announced the launch of impeachment proceedings Tuesday, alleging that Trump had betrayed his country in seeking Ukraine's help to find dirt on Biden, the leading Democratic candidate for the November 2020 presidential election.

The White House was scrambling though to defend Trump, who faces the possibility of becoming the third US president in history to be impeached.

In New York to attend the United Nations General Assembly, Trump branded the impeachment probe the "single greatest witch hunt in American history... a disgraceful thing."

The White House was stunned when Pelosi, the powerful speaker of the House of Representatives, suddenly abandoned months of resistance and announced the impeachment investigation.

Only two presidents in US history have been impeached, Andrew Johnson in 1868 and Bill Clinton in 1998, deeply tarring both of their legacies.

- 'Betrayal' -

Pelosi declared the formal investigation 11 days after the news that an anonymous US intelligence official had filed a formal whistleblower complaint on Trump's alleged double-dealing with Ukraine, tying aid to the country for dirt on Biden.

"The actions of the Trump presidency revealed the dishonorable facts of the president's betrayal of his oath of office, betrayal of our national security, and betrayal of the integrity of our elections," she said.

With the call transcript not a verbatim record, Democrats made clear its release would not satisfy their investigation into whether the US leader broke the law.

They demanded the full release of the whistleblower complaint, even as the Justice Department declared that it showed no wrongdoing and said "no further action was warranted."

Pelosi had resisted pressure from the party rank-and-file for impeachment, preferring to focus their energies on the presidential and congressional elections next year as the public has shown little taste for a full-blown congressional probe into Trump.

A Quinnipiac poll released Wednesday said that only 37 percent of voters support impeaching Trump while 57 percent oppose, even if more than half disapprove of the US leader's performance.

But Pelosi said there was now strong evidence of Trump's wrongdoing.

"No. You *don't* ask foreign governments for help in our elections. That *is not* right," she tweeted Wednesday.