A senior Republican congressman has called on the FBI to hand over records of former director James Comey’s contacts with President Donald Trump.
House Oversight Committee chair Jason Chaffetz demanded that all correspondence be presented by 24 May.
US media have quoted a memo by Mr Comey that reportedly says Mr Trump asked him to drop an inquiry into links between his ex-national security adviser Michael Flynn and Russia.
The White House denied the allegation.
“The president has never asked Mr Comey or anyone else to end any investigation, including any investigation involving General Flynn,” it said.
Mr Trump fired Mr Comey last week in a move that stunned Washington.
Meanwhile Russian President Vladimir Putin promised to release to Congress records of a White House meeting between Mr Trump, Russian Foreign Minister Sergey Lavrov and Russian Ambassador Sergey Kislyak.
Mr Putin said Mr Trump had passed on no secrets at the meeting.
Critics have questioned the timing of the meeting, which came a day after Mr Comey was sacked, and on Tuesday US media quoted officials as saying Mr Trump had passed on classified information there.
‘Subpoena pen ready’
During a White House meeting with Mr Comey in February, the alleged memo quotes Mr Trump as referring to Mr Flynn and saying: “I hope you can let this go.”
The memo was written immediately after the meeting, a day after Mr Flynn resigned, according to media reports.
In a letter to acting FBI director Andrew McCabe, Mr Chaffetz said the memo, along with others made by Mr Comey from his meetings with the president, “raise questions as to whether the president attempted to influence or impede the FBI’s investigation”.
He adds that the FBI should “provide, no later than May 24, 2017, all memoranda, notes, summaries and recordings referring or relating to any communications between Comey and the president”.
Mr Chaffetz had earlier tweeted that he would force the FBI to comply if necessary.
“@GOPoversight is going to get the Comey memo, if it exists. I need to see it sooner rather than later. I have my subpoena pen ready,” he said.
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 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 Mr 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.
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”.
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.”