The US House of Representatives will launch a formal impeachment inquiry into President Donald Trump over reports he sought foreign help to smear a political rival.

The move sets up a clash between Congress and the White House that could spill into the 2020 presidential election campaign.

House Speaker Nancy Pelosi announced the inquiry yesterday after a closed-door meeting with Democratic politicians, saying President Trump's actions appear to have undermined national security and violated the US Constitution.

"The president must be held accountable. No one is above the law," said Ms Pelosi, who had for months been reluctant to agree to an impeachment effort.

President Trump responded on Twitter, calling the inquiry "Witch Hunt garbage."

Ms Pelosi's change of heart followed reports that Mr Trump had pressured Ukrainian President Volodymyr Zelenskiy in a 25 July phone call to investigate Democratic presidential front-runner Joe Biden and his son.

President Trump has said that he will release a transcript of his phone call.

He also confirmed that he had withheld nearly $400m in US aid to Ukraine but denied he did so as leverage to get Mr Zelenskiy to initiate an investigation that would damage Mr Biden.

Ms Pelosi said the six congressional committees currently investigating President Trump would continue with their probes as part of the inquiry.

Mr Biden has called on Mr Trump to fully comply with congressional investigations into the matter or risk impeachment.

"If he continues to obstruct Congress and flout the law, Donald Trump will leave Congress in my view with no choice but to initiate impeachment proceedings," Mr Biden told reporters in Wilmington, in his home state of Delaware.

President Trump, who has withstood repeated scandals since taking office in January 2017, said a "complete, fully declassified and unredacted" transcript of the 25 July call would be released on Wednesday.

The controversy came to light after a whistleblower from within the US intelligence community lodged a complaint with an internal watchdog about Mr Trump's conversation with Mr Zelenskiy.

House Intelligence Committee Chairman Adam Schiff said his panel is communicating with a lawyer representing the whistleblower and that the individual would like to testify this week.

President Trump said the transcript would show the call was "totally appropriate," that he had not pressured Mr Zelenskiy to investigate Mr Biden and that there had been no "quid pro quo" for US aid in exchange for a probe.

170 House Democrats in support of impeachment probe

Launching an impeachment inquiry is a politically perilous step 14 months before the 2020 election.

Ms Pelosi, who was credited with winning back control of the House of Representatives in 2016, had until now stiffly resisted the move, hoping to keep the focus on taking over the Senate and White House in 2020.

But after seven moderate Democrats declared themselves in support of a full-blown impeachment probe yesterday, analysts counted at least 170 of the party's 235 House members in support.

"The president of the United States may have used his position to pressure a foreign country into investigating a political opponent, and he sought to use US taxpayer dollars as leverage to do it," the seven, including Afghanistan and Iraq wars veteran Jason Crow and ex-CIA agent Abigail Spanberger, wrote in the Washington Post.

"This flagrant disregard for the law cannot stand."