President Donald Trump asked FBI chief James Comey to drop an inquiry into links between his ex-national security adviser and Russia, US media reports.
"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.
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".
Section 1512 requires a person not only to attempt to obstruct justice but to do it with "corrupt" intent, and legal experts have told the Washington Post
that is not clear in this case.