US House Speaker Nancy Pelosi announced Tuesday the opening of a formal impeachment inquiry against President Donald Trump, saying he had violated the constitution in seeking help from Ukraine to hurt Democratic opponent Joe Biden.

This combination of pictures created on September 24, 2019 shows US Speaker of the House Nancy Pelosi and US President Donald Trump. [Photo: AFP]

"The actions of the Trump presidency revealed (the) dishonorable fact of the president's betrayal of his oath of office, betrayal of our national security and betrayal of the integrity of our elections," she said.

"Therefore, today I'm announcing the House of Representatives moving forward with an official impeachment inquiry."

Pelosi spoke after meeting fellow members of the Democratic House caucus, where impeachment calls are surging despite concerns by leadership that the move could hurt their chances in the 2020 election.

The announcement was sparked by a scandal over Trump's reported attempt to pressure the incoming president of Ukraine to open a corruption investigation into his main challenger for the White House, Joe Biden, and Biden's son Hunter.

Triggering the confrontation was an as yet secret whistleblower complaint centered on Trump's phone call July 25 with Ukrainian President Volodymyr Zelensky.

The White House has refused to release to Congress the secret complaint from an intelligence community whistleblower that reportedly depicts Trump seeking help from Ukraine to fight Biden, currently the Democratic Party's frontrunner for the presidential nomination.

Trump has admitted he spoke to Zelensky about Biden in the call, and said Tuesday he had halted aid to the country temporarily, before it was unfrozen last week.

But he rejected charges that the two were tied, saying the aid freeze was to prod European countries to increase their support for the Ukraine government.

Pelosi had been stiffly resisting impeachment, hoping to keep the focus on capturing the Senate and White House in 2020.

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

"I was there... in the '90s when we wrote the whistleblower laws and continued to write them to ensure the security of our intelligence and the safety of our whistleblowers," Pelosi said.

"I know what their purpose was. And we proceeded with balance and caution as we wrote the laws.

"I can say with authority, the Trump administration's actions undermine both -- our national security and our intelligence -- and our protections of the whistleblowers, more than both."