US Democrats have opened a formal impeachment inquiry into President Donald Trump over allegations he sought help from a foreign power to damage a political rival.
Top Democrat Nancy Pelosi said the president "must be held accountable".
Mr Trump denies impropriety and called the efforts "witch hunt garbage".
While there is strong support from Democrats on impeachment, if the inquiry moves forward it is unlikely to pass the Republican-controlled Senate.
The row was sparked by reports an intelligence whistleblower lodged a formal complaint about a phone call President Trump made with Ukrainian counterpart Volodymyr Zelensky.
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 former Vice-President Joe 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.
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."
As House Speaker Ms Pelosi is the most senior Democrat. 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 denied wrongdoing and 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." He is the current frontrunner to take on Mr Trump in the 2020 election.
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 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.
According to the New York Times, the whistleblower has expressed an interest in talking to lawmakers and the White House and intelligence officials are working on a deal to allow the unnamed person to speak to Congress.
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