Speaker of the House Nancy Pelosi, D-Calif., is questioned by reporters as she departs the Capitol en route to a speaking event in Washington, Tuesday, Sept. 24, 2019. Pelosi will meet with her caucus later as more House Democrats are urging an impeachment inquiry amid reports that President Donald Trump pressured Ukraine to investigate former Vice President Joe Biden and his family. (AP Photo/J. Scott Applewhite)

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 fight between Democrats and the president ahead of the 2020 election.