The U.S. House of Representatives will launch a formal impeachment inquiry into U.S. President Donald Trump over reports he sought foreign help to smear Democratic frontrunner Joe Biden.

Pelosi confirms inquiry

House Speaker Nancy Pelosi announced the inquiry on Tuesday, saying Trump's actions appeared to have undermined national security and violated the U.S. Constitution.

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

Trump fired back quickly on Twitter, calling the inquiry "Witch Hunt garbage."

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

Trump to release transcript

Trump promised on Tuesday to release a "complete, fully declassified and unredacted" transcript of his phone call.

The president has acknowledged he discussed Biden in the call, but denied he withheld nearly 400 million U.S. dollars in aid to Ukraine as leverage to persuade Zelensky to launch a probe that would damage Biden, who leads opinion polls in the Democratic race to face Trump in the November 2020 election.

Details from phone call aren't likely to come from a recording or be verbatim, former White House and national security officials told Reuters. Instead, because of standard White House protocol for handling phone calls between the president and other world leaders, a transcript is likely to be put together from written notes by U.S. officials who listen in.

Biden said he would back impeachment if the president did not fully comply with congressional investigations. "If we allow a president to get away with shredding the Constitution, that will last forever," Biden told reporters in Wilmington, in his home state of Delaware.

Most Democratic presidential contenders support an impeachment inquiry.

Whistleblower may testify

Democrats are also seeking the original complaint about Trump's call, filed by a whistleblower within the U.S. intelligence community, as well as information on deliberations over the Ukrainian aid.

House Intelligence Committee Chairman Adam Schiff said his panel was communicating with an attorney representing the whistleblower and that the individual would like testify this week.

The U.S. Senate approved a resolution on Tuesday calling on the Ukraine whistleblower complaint to be submitted to the Senate and House Intelligence committees. Trump administration officials so far have refused to let the complaint be submitted.

The impeachment inquiry could eventually lead to Trump's removal from office. But even if the Democratic-controlled House voted to impeach Trump, the Republican-controlled Senate would have to take the next step of removing him from office after a trial.

It will be the first impeachment inquiry in Congress since the 1998 probe of President Bill Clinton for perjury and obstruction of justice in relation to his affair with White House intern Monica Lewinsky.

(With input from Reuters)