The US Democratic Party has begun a formal impeachment inquiry into President Donald Trump over allegations that he pressured a foreign power to damage a political rival.

Top Democrat Nancy Pelosi said the president "must be held accountable".

Mr Trump has denied wrongdoing and called the efforts a "witch hunt".

There is strong support from House Democrats for impeachment, but the proceedings would be unlikely to pass the Republican-controlled Senate.

The high-stakes move by House Speaker Ms Pelosi, prompted by allegations that Mr Trump pressured the Ukrainian president to investigate his leading political rival Joe Biden, lays the groundwork for a bitter and potentially hugely consequential confrontation between Democrats and the president ahead of the 2020 election.

If the inquiry moves forward the House of Representatives will vote on any charges and, with the Democrats in the majority, the vote would likely be carried - making Mr Trump the third president in US history to have been impeached.

But the proceedings would likely then stall in the Senate, where the president's Republican Party holds enough seats to prevent him from being removed from office by a two-thirds majority.

Ms Pelosi did not provide any timeline for how the process might play out.

How did we get here?

Senior democrats including Ms Pelosi had previously resisted growing calls from within the party to begin impeachment proceedings. But the party's leadership united on the issue after an intelligence whistleblower lodged a formal complaint about one or more phone calls between President Trump and Ukrainian President Volodymyr Zelensky.

The Trump administration has so far refused to release the whistleblower complaint to Congress but Democrats say Mr Trump threatened to withhold military aid to Ukraine unless Mr Zelensky agreed to investigate unsubstantiated corruption allegations against Mr Trump's leading political rival Joe Biden and his son Hunter.

Mr Trump has admitted discussing Joe Biden with Mr Zelensky but denied that he exerted pressure on the Ukrainian president to investigate his political rival.

On Tuesday, Mr Trump confirmed that military aid from Ukraine had been withheld but said he had done this to try and pressure European nations to increase their contributions to the country.

Two presidents have been impeached in US history - Andrew Johnson in 1868, and Bill Clinton in 1998. But neither were removed from office by a Senate trial.

Richard Nixon resigned in August 1974 before he could be impeached.