Washington, 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".
A BBC News report on Wednesday said 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.
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.
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.
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.
For months now, Democratic leaders in the House of Representatives have been playing a semantics game. They wanted those who supported and those who opposed a formal impeachment inquiry into President Donald Trump to both think they were getting what they wanted.
This strategy suggested a fear by Speaker Nancy Pelosi and others that heading down the path to impeachment would put moderate Democrats facing tough 2020 re-election fights at risk.
That calculation appears to have changed, after the rapid drumbeat of new revelations about Mr Trump's contacts with Ukrainian President Volodymyr Zelensky. 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 - at the very least - being open to it.
The path forward is uncertain. The president has announced that he will release the transcript of his 25 July phone conversation with Zelensky. While that won't be enough for Democrats, perhaps the White House will do more to accede to Congress's requests.
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, 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 statement, lawyers representing the whistleblower called on Mr Maguire to hand over the whistleblower complaint to Congressional intelligence committees.
The unnamed person could end up meeting lawmakers directly and the White House is reportedly working on a deal to make this happen.
What happens next?
Ms Pelosi's announcement gives an official go-ahead for a committee to investigate the US president's phone call with the Ukrainian leader and determine whether he committed an impeachable offence.
In her announcement she said the six other congressional committees investigating Mr Trump on other matters would continue under the umbrella of a formal impeachment inquiry.
What would impeachment take?
If it 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.
A YouGov poll said 55pc of Americans would support impeachment if it was confirmed that President Trump suspended military aid to Ukraine in order to push the country's officials to investigate Joe Biden.