President Donald Trump asked FBI chief James Comey to drop an inquiry into links between his ex-national security adviser and Russia, US media report.
"I hope you can let this go," Mr Trump reportedly told Mr Comey after a White House meeting in February, according to a memo written by the ex-FBI director.
The memo was written immediately after the meeting, a day after Michael Flynn resigned, according to media reports.
The White House has denied the allegation in a statement.
"The president has never asked Mr Comey or anyone else to end any investigation, including any investigation involving General Flynn," it said.
An influential Republican congressman has called for the FBI to hand over all relevant documents within a week.
Jason Chaffetz, who chairs the House Oversight Committee, demanded all correspondence relating to communications between Mr Comey and the president be presented by 24 May.
Mr Flynn was forced out in February after he misled the vice-president about his conversations with Russia's ambassador before Mr Trump took office.
The latest Russian twist, first reported by the New York Times, comes a week after Mr Trump fired Mr Comey over his handling of the FBI investigation into Hillary Clinton's use of a private email server while at the state department.
Mr Comey's dismissal sent shockwaves through Washington, with critics accusing the president of trying to thwart the FBI investigation into Russia's alleged interference in the US election and any Moscow ties to Trump associates.
What's the latest allegation?
Mr Comey reportedly wrote a memo following a meeting with the president on 14 February that revealed Mr Trump had asked him to close an investigation into Mr Flynn's actions.
He reportedly shared the memo with top FBI associates.
"I hope you can see your way clear to letting this go, to letting Flynn go," the president told Mr Comey, according to the memo. "He is a good guy."
Mr Comey did not respond to his request, according to the memo, but replied: "I agree he is a good guy."
In response to the report, a White House official pointed out that acting FBI director Andrew McCabe had testified last week that there had been "no effort to impede our investigation to date".
The I-word surfaces - Anthony Zurcher, BBC News, Washington
The "I" word - impeachment - has already been broached by politicians as moderate as independent Senator Angus King of Maine. If this were a Democratic-controlled House of Representatives, articles of impeachment would likely be in the drafting process.
Republicans still call the shots in Congress, however, and it's a significant leap to get them to abandon the Trump presidency and any hope of advancing their agenda for the foreseeable future.
But some, like Senator John McCain - who said this has become a scandal of "Watergate size and scale" - are clearly wavering.
The former Republican presidential nominee is a bit of a wild card, of course.
For the rank-and-file to turn on the president will require them to admit their complicity in a failed presidency.
Why was Flynn being investigated?
Mr Flynn's departure in February came months after suspicions were raised among intelligence officials.
He resigned as White House national security adviser after just 23 days on the job over revelations that he had discussed lifting sanctions on Moscow with Russian ambassador Sergei Kislyak, before Mr Trump was sworn in.
It is illegal for private citizens to conduct US diplomacy.
Since Mr Flynn stepped down, the Pentagon has launched an investigation into whether he failed to disclose payments from Russian and Turkish lobbyists for speeches and consulting work.
Mr Flynn's Russian ties are under investigation by the FBI and the House and Senate Intelligence Committees, as part of wider inquiries into claims Moscow sought to tip the election in favour of Mr Trump.
Has Trump obstructed justice?
Adam Schiff, the highest ranked Democrat on the House Intelligence Committee, said this intervention by Mr Trump, if correct, amounted to "interference or obstruction of the investigation".
Senator John McCain reportedly said at a dinner that the Trump scandals had now reached "Watergate size".
The key legal statute is 18 US Code Section 1512, which contains a broad definition allowing charges to be brought against someone who "obstructs, influences, or impedes any official proceeding, or attempts to do so".
It has been pointed out that Mr Trump did have the legal authority to fire Mr Comey, but there is a legal precedent for otherwise lawful acts to be considered an obstruction of justice if done with corrupt intentions, the New York Times says.
Legal experts have told the Washington Post that that is not clear in this case as intent is difficult to prove.
However, former federal prosecutor Samuel Buell told the Times: "The evidence of improper purpose has gotten much stronger since the day of Comey's firing.
"Trump has made admissions about that. And we now have evidence that he may have indicated an improper purpose previously in his communications with Comey about the Russia investigation."
What is the US media saying?
An opinion piece in the New York Times, which first reported on Mr Comey's memo, expresses frustration with Republican lawmakers.
"When will Republicans in Congress decide that enough is enough?" it reads. "Do they need Mr. Trump's approval ratings to dip below 30%? Do they need first to ram through their deeply unpopular agenda? Or it is possible they might at last consult their consciences, and recall that they took an oath to uphold the Constitution?"
Dan Balz in the Washington Post says the Republican strategy of hoping the storm would pass has run aground, though many will continue to resist.
"But the double revelations of the past two days show that events are forcing a change in everyone's calculations," he writes.
Politico says Republicans are in a bind because the crisis could threaten their entire legislative programme.
"If Republicans are paralysed and can't pass anything despite control of the White House and Congress, how can they justify their majorities when they go before voters next year?" it says.
Meanwhile Tucker Carlson on Fox News urged viewers not to take everything they read at face value. He said "an awful lot of people in Washington" wanted to remove the president.
"What are their motives? In a lot of cases those are completely unknown. A lot of what you hear and accept uncritically you ought to question." he said.