US House of Representatives Speaker Nancy Pelosi has announced a formal investigation that could lead to impeachment proceedings against President Donald Trump.

Key points: The allegations centre on a July phone call between the US President and his Ukrainian counterpart

The allegations centre on a July phone call between the US President and his Ukrainian counterpart Ms Pelosi called Mr Trump's actions "a betrayal of national security"

Ms Pelosi called Mr Trump's actions "a betrayal of national security" Mr Trump tweeted "PRESIDENTIAL HARASSMENT!" in response

The inquiry centres on whether Mr Trump abused his presidential powers and sought help from a foreign government for his re-election.

Ms Pelosi, the most senior elected Democrat, had previously been reluctant to back moves towards impeachment, but there had been mounting pressure from her colleagues.

"The President must be held accountable. No-one is above the law," she said after meeting with her Democratic colleagues.

"The actions taken to date by the President have seriously violated the Constitution, especially when the President says, 'Article 2 says I can do whatever I want'.

"And this week, the President has admitted to asking the President of Ukraine to take actions which would benefit him politically.

"The actions of the Trump presidency revealed the dishonourable fact of the President's betrayal of his oath of office, betrayal of our national security, and betrayal of the integrity of our elections."

Democrats allege Mr Trump asked Ukrainian President Volodymyr Zelenskiy during a July phone call to investigate former vice-president and Democratic presidential hopeful Joe Biden in return for tens of millions of dollars in military aid.

A whistleblower from within the US intelligence community lodged a complaint about the conversation, and both The Wall Street Journal and The New York Times reported on its contents on Friday.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 36 seconds 36 s President Donald Trump has called the allegations against him as 'ridiculous' and a 'witch hunt'

Mr Trump responded to the news of the impeachment inquiry on Twitter:

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Earlier, he denied the allegations and said he had authorised the release of a transcript of the call on Wednesday (local time).

"You will see it was a very friendly and totally appropriate call," he said.

Donald Trump says he has authorised the release of a transcript of the call in question. ( AP: Pablo Martinez Monsivais )

Mr Trump, who is in New York meeting with world leaders at the United Nations, also said that Democrats would lose next year's elections if they pursued impeachment.

"If she does that, they say that's a positive for me," he said.

Earlier this week, he labelled Mr Biden and his son Hunter — who worked for a company drilling for gas in Ukraine — as "corrupt".

But neither the President, nor his aides, have provided any details of the allegations.

Reports about Mr Trump's call with the Ukrainian President allege that he repeatedly asked Mr Zelenskiy to investigate whether Mr Biden misused his position as vice-president under former president Barack Obama to threaten to withhold US aid unless a prosecutor, who was looking into the gas company in which Mr Biden's son was involved, was fired.

Mr Biden has confirmed he wanted the prosecutor fired but denies it was to help his son, saying the wider US government, the European Union and other international institutions also wanted the prosecutor fired for his alleged failure to pursue major corruption cases.

The announcement of a formal impeachment inquiry — which will investigate whether there should be a vote to impeach the President — does not have the support of US House Republican leader Kevin McCarthy.

"Speaker Pelosi happens to be the Speaker of this House, but she does not speak for America when it comes to this issue," he said.

Republican Senate Majority Leader Mitch McConnell also released a statement in support of Mr Trump.

"Washington Democrats have been searching for ways to reverse their 2016 election defeat since before President Trump was even inaugurated," the statement read.

"House Democrats' priority is not making life better for the American people, but their nearly three-year-old fixation on impeachment."

Ms Pelosi's decision seems certain to exacerbate the United States' fierce partisan divides and inject deep uncertainty into the 2020 presidential contest.

ABC/wires