US House of Representatives Speaker Nancy Pelosi has announced a formal impeachment inquiry into US President Donald Trump.

The most senior Democrat in Congress said the process was triggered by allegations over Trump pressuring the Ukrainian president to investigate his potential presidential rival Joe Biden in return for military aid.

In a televised statement, she called his conduct a “violation of the law”.

“The president must be held accountable. No one is above the law,” said Pelosi.

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

Pelosi had been the biggest obstacle to opening a full impeachment inquiry by a vote in the House of Representatives.

She had previously argued that a fully empowered impeachment inquiry into Trump would be divisive, distract from the Democrats’ agenda and force recently elected politicians to take a vote that could upset swing voters.

But then came news that Trump pressured Ukrainian President Volodymyr Zelensky to open an investigation into 2020 Democratic presidential candidate Biden and the former vice president’s son Hunter Biden while withholding military aid approved for the country. 

This request for foreign intervention in the 2020 election, and the potential act of bribery, pushed Pelosi over the edge.

After the new revelations about the president’s wrongdoing emerged, more and more Democratic lawmakers joined calls for Congress to open a full impeachment inquiry.

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

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

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

Quid pro quo is a Latin phrase meaning a favour that is exchanged for a favour.

Calls for impeaching Trump began in earnest after the April 18 release of special counsel Robert Mueller’s report on Russian interference in the 2016 election.

While the report did not find any criminal acts in Trump’s and his campaign’s efforts to encourage Russian interference in the election, it did outline multiple efforts made by the president to obstruct the investigation.

Biden on Tuesday called on 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,” Biden told reporters in Wilmington, in his home state of Delaware.