US President Donald Trump asked former FBI Director James Comey to shut down an investigation into ousted national security adviser Michael Flynn, it has been claimed.
According to The New York Times, Mr Trump made the request during a meeting at the White House in February.
The newspaper cited a memo Mr Comey wrote shortly after the conversation.
It said the president told Mr Comey: "I hope you can let this go."
The Flynn investigation was part of a broader probe into Russian interference in last year's presidential election, during which Mr Flynn was an adviser to the president's campaign.
Mr Flynn, a retired general, resigned a day before the 14 February meeting, after it emerged he misled US officials about his contacts with Russia.
The president fired Mr Comey last week, saying he did so based on his very public handling of the Hillary Clinton email probe and how it affected his leadership of the FBI.
The Republican Chairman of the House Oversight Committee, Jason Chaffetz, has given the FBI until 24 May to hand over all documents and recordings that detail communications between Mr Comey and the president.
ITV News Washington Correspondent Robert Moore said "Round One of Comey v Trump is tonight underway".
The New York Times said Mr Trump’s request is the "clearest evidence" that the president has "tried to directly influence" the Justice Department and FBI investigation into links between his associates and Russia.
The White House has denied the report.
In a statement, it said: "While the President has repeatedly expressed his view that General Flynn is a decent man who served and protected our country, the President has never asked Mr Comey or anyone else to end any investigation, including any investigation involving General Flynn.
"The President has the utmost respect for our law enforcement agencies, and all investigations.
"This is not a truthful or accurate portrayal of the conversation between the President and Mr Comey."
- What we know about the Comey memo
The New York Times reported on Tuesday that Mr Comey wrote the memo detailing his conversation with the president immediately after the meeting, which took place the day after Mr Flynn resigned, according to two people who read the memo.
During the discussion, Mr Trump allegedly told the former FBI Director: “I hope you can see your way clear to letting this go, to letting Flynn go.
“He is a good guy. I hope you can let this go.”
According to the newspaper, the memo was part of a paper trail Mr Comey had created documenting what he perceived as the president's improper efforts to influence an ongoing investigation.
It added that Mr Comey apparently shared the memo with senior FBI officials and close associates.
The New York Times said it has not viewed a copy of the memo, but one of Mr Comey’s associates read parts of it to one of its journalists.
The FBI has declined to comment.
- Why it matters
The apparent record of Mr Comey's conversation with the president matters because an FBI agent's contemporaneous notes are widely held up in court as credible evidence of conversations.
Legal experts have taken a dim view of Mr Trump's alleged comments.
Erwin Chereminsky, a constitutional law professor at the University of California, said: "For the president to tell the FBI to end a potential criminal investigation, that's obstruction of justice."
"This is what caused President Nixon to resign from office."
However he added that intent was a critical element of an obstruction of justice charge, and the president's words could be subject to interpretation and possibly put into the context of other actions, like the sacking of Mr Comey.
- The reaction
A number of politicians have demanded strong action.
Mark Warner of Virginia, top Democrat on the Senate intelligence committee, said he would ask Mr Comey for additional material as part of the panel's investigation. "Memos, transcripts, tapes - the list keeps getting longer," he said.
John McCain, chairman of the Senate Armed Services Committee, said the Trump-Russia reports were "deeply disturbing" and said they could impede allies' willingness to share intelligence with the US
Mitch McConnell, the Republican leader of the Senate, said he was not losing confidence in Mr Trump as president but added, "It would be helpful to have less drama emanating from the White House."
- A bad week for Trump
The latest revelation came just 24 hours after another report in the US media, which claimed that Mr Trump had shared highly classified information with Russia about Islamic State militants during a meeting at the White House.
The Washington Post cited current and former US officials, who said the president jeopardised a critical source of intelligence on IS in his conversations with the Russian foreign minister and the Russian ambassador to the US.
White House officials initially denied the report, then claimed that Mr Trump didn't even know the source of the information himself.
The president later took to Twitter to defend his "absolute right" to open dialogue with the Kremlin.