Speaker of the House Nancy Pelosi has announced Congress is launching a formal impeachment inquiry of U.S. President Donald Trump.

"Today, I am announcing the House of Representatives moving forward with an official impeachment inquiry. The president must be held accountable. No one is above the law," she said, according to CNN.

President Trump took to Twitter just minutes after Speaker Nancy Pelosi announced the start of a formal impeachment inquiry by the House, calling it "more breaking news Witch Hunt garbage."

"Such an important day at the United Nations, so much work and so much success, and the Democrats purposely had to ruin and demean it with more breaking news Witch Hunt garbage. So bad for our Country!" he tweeted.

According to Reuters, 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 Hunter, who had worked for a company drilling for gas in Ukraine.

Trump promised on Tuesday to release a transcript of his phone call.

Read alsoTrump authorizes release of "complete, fully declassified" transcript of phone call with Zelensky

He has acknowledged he discussed Biden in the call, but denied he withheld nearly $400 million in U.S. aid to Ukraine as leverage to get 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.

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

Only three presidents have been subjected to formal impeachment proceedings while in office, none as they were facing re-election.