US Democrats have opened a formal impeachment inquiry into President Donald Trump over claims that he sought political help from Ukraine.
Announcing the move top Democrat Nancy Pelosi said the president “must be held accountable”.
Mr Trump has denied impropriety and called the efforts “garbage”.
While there is strong support from Democrats on impeachment, if the inquiry moves forward it is unlikely to pass the Republican-controlled Senate.
What is this row about?
Last week reports said US intelligence officials had complained to a government watchdog about Mr Trump’s interactions with a foreign leader, who was later revealed to be the president of Ukraine, Volodymyr Zelensky.
That whistleblower’s complaint, deemed “urgent” and credible by the intelligence inspector general, has been demanded by Democrats but the White House and Department of Justice have refused to provide it.
What exactly was said remains unclear but Democrats accuse Mr Trump of threatening to withhold military aid to force Ukraine to investigate corruption allegations against Mr Biden and his son Hunter.
Mr Trump has acknowledged discussing Joe Biden with Mr Zelensky but said he was only trying to get Europe to step up assistance by threatening to withhold military aid.
What did Ms Pelosi say?
Ms Pelosi said Mr Trump had committed “a violation of the law”, and 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.”
She has so far resisted calls among her liberal rank-and-file to attempt to remove the Republican president from office as such an effort could bolster his support.
Mr Biden has also backed 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.” The former vice-president is the current frontrunner to take on Mr Trump in the 2020 election.
How has Mr Trump responded?
In a series of tweets Mr Trump said Democrats “purposely had to ruin and demean” his trip to the UN “with more breaking news Witch Hunt garbage”.
“They never even saw the transcript of the call. A total Witch Hunt!” he added.
He has promised to release a transcript of his conversation with Ukraine’s president 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.
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 any charges and with the Democrats in the majority there, it could comfortably pass.
But it would next move to the Senate, where a two-thirds majority is required – and where the Republicans hold sway.
A YouGov poll said 55% 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.