US President Donald Trump has denied a whistle-blower's claim of wrongdoing in a call with a foreign leader, amid reports he pressured Ukraine's president to investigate the son of Democratic political rival Joe Biden, who demanded the transcript of the call released immediately.
The whistle-blower's complaint has triggered a tense showdown between the US Congress, whose Democratic leaders are demanding the review of the complaint, and the executive branch which has barred them from doing so.
It has also raised concerns Trump sought to strong-arm Ukraine into providing damaging information on the president's possible 2020 challenger, which would represent dangerous foreign meddling in the US election similar to the interference blamed on Russia in 2016, when Trump defeated Democratic rival Hillary Clinton.
According to a report by The Washington Post, which cited two unnamed former US officials, the complaint stemmed from Trump's communications with Ukraine, and a "promise" allegedly made by the president.
Suspicions have focused on a July 25 call between Trump and Ukraine's President Volodymyr Zelensky, a former comedian who assumed office in May.
Trump's Democratic opponents have been probing that call in connection with allegations Trump and his personal lawyer Rudy Giuliani tried to pressure the Ukrainian administration.
The Wall Street Journal meanwhile reported that Trump pressed Zelensky about eight times on the call to investigate possible corruption involving Biden's son Hunter, who worked with a Ukrainian natural gas company.
Trump on Friday defended himself, angrily declaring that the "ridiculous" story came from a "partisan whistle-blower", even though he did not know the person's identity.
Trump insisted he had a "totally appropriate" conversation, without detailing with whom, or what was said.
When asked by reporters whether he discussed the Bidens with Zelensky, Trump replied: "It doesn't matter what I discuss."
He did not deny having done so, but instead urged media to "look into" Biden's involvement in Ukraine, and his videotaped comments last year in which the ex-US vice president said the former President Barack Obama's administration would freeze $1bn in US loan guarantees, unless Kiev did not fire its top prosecutor.
'Willingness to abuse power'
Biden shot back forcefully, demanding Trump immediately release the transcript of the call and saying the reports that Trump sought to coerce his Ukrainian counterpart, if true, amounted to "clear-cut corruption".
"If these reports are true, then there is truly no bottom to President Trump's willingness to abuse his power and abase our country," he said in a statement.
Kimberly Halkett, reporting from the White House, said that Trump vigorously downplayed the allegations that he abused his position of power during the phone conversation with Zelensky.
"However, Democrats allege Trump may have tried to help his presidential re-election campaign by pressuring Kiev to investigate business deals between Ukrainian companies and Biden's son, while Biden was vice president," she said.
The whistle-blower scandal mushroomed this week when House intelligence chairperson Adam Schiff revealed that the acting Director of National Intelligence, Joseph Maguire, refused to hand over the complaint to Congress.
"The whole purpose is being frustrated here because the director of national intelligence has made the unprecedented decision not to share the complaint with Congress," he told reporters.
Halkett reported that after initially dodging Congressional requests, Maguire is scheduled to publicly testify before Schiff's committee on Thursday about why the executive branch wants the complaint hidden from the Congress.
Amid the swirling allegations, Giuliani told CNN during a rambling interview on Thursday that "of course I did" ask Ukraine to investigate Biden.
Giuliani said he did not know whether Trump brought up Biden, but said he would have "every right" to do so.
Adding yet another layer to the controversy, congressional policymakers were concerned the Trump administration had been holding back military aid to Ukraine at the time. The aid was finally released last week.
Zelensky inherited an armed conflict against pro-Russian rebels that has killed nearly 13 000 in five years and caused serious economic difficulties.
Democratic Congress members say a president conditioning foreign aid on providing dirt on a US election rival would amount to grounds for impeachment.
"The president can't use US aid as a weapon to serve his own political purposes," House Democrat Seth Moulton tweeted. "This is an impeachable offence if true."
Clinton herself weighed in Friday with a sharp critique.
"The president asked a foreign power to help him win an election. Again," she tweeted.
The scandal has set Washington on edge, with House Speaker Nancy Pelosi saying the whistle-blower complaint raises "grave, urgent concerns for our national security".
As the crisis swells, Trump and Zelensky will meet for the first time on Wednesday, during the United Nations General Assembly.