US Speaker Nancy Pelosi has announced an impeachment inquiry into Donald Trump’s controversial phone call with Ukraine’s president, warning that “no-one is above the law”.

The dramatic development comes after Mr Trump admitted to withholding aid to Ukraine after which it’s claimed he pressed its president to investigate key political opponent Joe Biden’s son, who worked in the country’s gas industry.

Ms Pelosi said on Wednesday Mr Trump’s actions may have “seriously violated the constitution” and she instructed the House of Representatives’ six committees to proceed with investigations.

“The president must be held accountable,” she announced.

“No-one is above the law.”

Mr Trump labelled the impeachment drive “presidential harassment” after earlier vowing to release “the complete, fully declassified and un-redacted” transcript of his call with president Volodymyr Zelenskysky.

Trump 2020 campaign fundraising text: pic.twitter.com/yklT6y3V6a

— Michelle Ye Hee Lee (@myhlee) September 24, 2019

Soon after the announcement, there were reports that Mr Trump’s team had sent out a text to his followers, looking to raise funds in the wake of the impeachment inquiry.

In a tweet, Mr Trump said the transcript would be released on Wednesday (local time) and would show the call was “totally appropriate” and there had been no quid pro quo for US aid in exchange for a probe into Mr Biden’s son.

They never even saw the transcript of the call. A total Witch Hunt!

— Donald J. Trump (@realDonaldTrump) September 24, 2019

It has also been revealed the whistleblower whose complaint sparked the latest scandal wants to testify and could be doing so as soon as this week, according to Adam Schiff, chairman of the House Intelligence Committee.

We have been informed by the whistleblower’s counsel that their client would like to speak to our committee and has requested guidance from the Acting DNI as to how to do so.

We‘re in touch with counsel and look forward to the whistleblower’s testimony as soon as this week.

— Adam Schiff (@RepAdamSchiff) September 24, 2019

Mr Trump is seeking re-election next year and Mr Biden, the former US vice president, is the front runner for the Democratic presidential nomination.

The Democrats have accused the president of using the $US400 million ($588 million) aid cuts as political leverage to smear a potential 2020 presidential opponent.

Mr Trump has admitted withholding aid to Ukraine but denies it was to press Ukraine’s president Volodymyr Zelensky to investigate former Vice President Joe Biden’s son, Hunter.

Mr Trump says he instructed the aid withdrawal because he wanted other European countries to contribute assistance.

“We’re putting up the bulk of the money, and I’m asking why is that?” he said earlier, adding: “What I want, and I insist on it, is that Europe has to put up money for Ukraine also.”

He has also admitted to discussing Joe Biden, saying on Sunday: “We had a great conversation.

“It was largely the fact that we don’t want our people, like Vice President Biden and his son, creating to the corruption already in the Ukraine.”

Earlier on Wednesday Ms Pelosi, a Democrat, said there did not need to be an overt ‘quid pro quo’ in the transcript of the conversation for an impeachable offence to have been committed.

Ms Pelosi has been trying to hold the line against impeachment for months, as her caucus remained deeply ambivalent about it.

But the Ukraine scandal has prompted at least 34 members — including several freshmen in vulnerable districts — to join calls for impeachment.

“If the President brings up he wants them to investigate something…of his political opponent, that is self-evident that it is not right,” she said.

“You don’t ask foreign governments to help us in our election. That’s what we try to stop with Russia. It’s wrong.

“And just bringing up the election is bad enough.

“That there would be a quid pro quo isn’t necessarily in the conversation, but in the sequencing. So this is not a good thing for a democracy.”

Mr Biden gave a statement earlier on Tuesday, saying that if Mr Trump continues to stonewall a House investigation, then Congress would have “no choice but to initiate impeachment.”

“I can take the political attacks. They’ll come and go and soon be forgotten,” Mr Biden said.

“But if we allow a president to get away with shredding the United States Constitution, that will last forever.”

-with AAP