House Speaker Nancy Pelosi announces official impeachment inquiry of Trump

US Speaker Nancy Pelosi has launched the House into a formal impeachment inquiry against President Donald Trump.

She gave in to mounting pressure from fellow Democrats and plunged a deeply divided nation into an election year clash between Congress and the commander in chief.

The probe centres on whether President Trump abused his powers and sought help from a foreign government for his re-election, actions Ms Pelosi said would mark a "betrayal of his oath of office".

She declared: "No one is above the law."

Ms Pelosi had long resisted pursuing impeachment, but her caucus moved swiftly in favour of a probe in recent days following reports that President Trump asked Ukraine's president to investigate Democratic foe Joe Biden and his son.

Her decision sets up her party's most urgent and consequential confrontation with a president who thrives on combat, and it injects deep uncertainty in the 2020 White House race.

President Trump reacted swiftly to the announcement.

Ahead of his meeting with world leaders at the United Nations, he tweeted that "the Democrats purposely had to ruin and demean it with more breaking news Witch Hunt garbage. So bad for our Country".

He added in further tweets: "They never even saw the transcript of the call. A total Witch Hunt."

At issue are President Trump's actions with Ukraine. In a summer phone call with Ukraine President Volodymyr Zelenskiy, he is said to have asked for help investigating Mr Biden and his son Hunter.

In the days before the call, President Trump ordered advisers to freeze 400 million US dollars in military aid for Ukraine — prompting speculation that he was holding out the money as leverage for information on the Bidens.

President Trump has denied that charge, but acknowledged he blocked the funds.

Ahead of Ms Pelosi's announcement, President Trump authorised the release of a transcript of his call with Ukraine's president, predicting it would show no evidence of wrongdoing.

The transcript is to be made public on Wednesday.

"You will see it was a very friendly and totally appropriate call," President Trump said.