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 potentially hugely consequential confrontation between Democrats and the president ahead of the 2020 election.

If the inquiry moves forward, the Democratic-controlled House of Representatives will vote on any charges. If Democrats were to remain united, the measure would be carried - and Mr Trump would become the third president in US history to be impeached.

But the proceedings would be expected to 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. He has denied that he exerted pressure on the Ukrainian president to investigate his political rival.

On Tuesday, Mr Trump confirmed that military aid to 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.

The dam has broken

For months now, Democratic leaders have been playing a semantics game. They wanted those who supported and those who opposed a formal impeachment inquiry into President Trump to both think they were getting what they wanted.

This strategy suggested Nancy Pelosi and others feared that heading down the path to impeachment would put moderate Democrats facing tough 2020 re-election fights at risk.

That calculation has changed after the rapid drumbeat of new revelations about Mr Trump's contacts with the Ukrainian president. Now even middle-of-the road politicians are coming out in favour of impeachment proceedings.

The dam has broken. The genie is out of the bottle. Pick your metaphor. The simple fact is that Ms Pelosi - a keen judge of the political mood within her caucus - has made the decision to shift from resisting impeachment to advocating for it.

The path forward is uncertain. Opinion surveys could show the latest drama is taking a toll on one party or the other, causing political will to crumble. Or, both sides could dig in for a long, gruelling battle that could drag into the darkest days of winter.

What did Nancy Pelosi say?

In a statement on Tuesday she said Mr Trump had betrayed his oath of office and committed "a violation of the law". She called his actions "a breach of his constitutional responsibilities".

"This week the president has admitted to asking the president of Ukraine to take action that would benefit him politically," she said, adding: "The president must be held accountable. No-one is above the law."

Mr Biden has denied wrongdoing and no evidence has emerged to back up the claims against him. He has also said he supports impeachment proceedings unless the US president complies with investigations.

Impeaching Mr Trump "would be a tragedy", Mr Biden said. "But a tragedy of his making." He is the current frontrunner to take on Mr Trump in the 2020 election.

How did Mr Trump and Republicans respond?

In a series of tweets Mr Trump said Democrats "purposely had to ruin and demean" his trip to the UN in New York "with more breaking news Witch Hunt garbage".

"They never even saw the transcript of the call. A total Witch Hunt!" he added.

He promised to release a transcript of his conversation with Ukraine's president on Wednesday to show it was "totally appropriate".

In his response to the Democrats' move, House Republican Leader Kevin McCarthy said: "Speaker Pelosi happens to be the Speaker of this House, but she does not speak for America when it comes to this issue."

"She cannot unilaterally decide we're in an impeachment inquiry," he added.

Meanwhile, the acting director of US national intelligence, Joseph Maguire, has refused to share the whistleblower report with Congress. He is due to testify before a public House intelligence committee hearing on Thursday.

In a separate development, the unnamed whistleblower is seeking to meet directly with lawmakers. House Intelligence Committee Chairman Adam Schiff suggested in a tweet that this could happen later this week.

What happens next?

Ms Pelosi's announcement gives an official go-ahead for lawmakers to investigate the US president's phone call with the Ukrainian leader and determine whether he committed an impeachable offence.

In her announcement Ms Pelosi said the six congressional committees already investigating Mr Trump would continue their work, but now under the umbrella of a formal impeachment inquiry.

If the process moves forward the House of Representatives will vote on one or more articles of impeachment. If any pass, the process would next move to the Senate, where a two-thirds majority is required - and where the Republicans hold sway.

Ukrainians ponder their president's role

By Vitaliy Shevchenko, BBC Monitoring

Commentators in Ukraine see President Volodymyr Zelensky's role in this affair as accidental, but at the same time believe it could lead to history-making consequences.

Den, a daily newspaper, warns President Zelensky against taking sides in the drama unfolding in Washington. "Ukraine is facing an unstoppable tsunami, and our president will just have to do some surfing," it says.

Some argue that the timing could not be worse for President Zelensky, who is scheduled to meet Donald Trump in New York later on Wednesday. Public TV station Pershy describes the controversy as a "trap" for Ukraine.

"It would be stupid to start playing into the hands of either Democrats or Republicans," said one of the channel's commentators.

Others contend that the Ukrainian president has US politicians over the barrel. "Zelensky has two pistols in his hands: one pointing at Trump, and the other at Biden," reports Pryamy TV.