President Donald Trump asked FBI chief James Comey to end 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 US media.
The White House has denied the report 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.
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 Russia 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 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 responded: “I agree he is a good guy.”
In response to the report, a White House official pointed out that acting FBI director Andrew McCabe testified last week that there “has been no effort to impede our investigation to date”.
A storm is brewing – Anthony Zurcher, BBC News, Washington
A person doesn’t rise as high as James Comey did in the federal government without learning how to cover his, er, posterior.
With this latest bombshell from the New York Times it’s clear that the former FBI director, who was unceremoniously sacked by the president, is poised to enjoy the last laugh.
At the moment the White House is denying Mr Comey’s reported characterisation of the conversation the two men had shortly after the president fired Michael Flynn.
In a he-said, he-said situation, however, the man who wrote contemporaneous memos – and that is memos, plural – will have the upper hand.
Add in that Mr Comey has a reputation for independence, and the face-off looks even more ominous for the president.
As deputy attorney general Mr Comey stood up to the Bush administration during a showdown over the legality of government surveillance programme. He also withstood withering criticism from Democrats over his handling of the Hillary Clinton email server investigation last year.
If he swears to a congressional committee that the president put undue pressure on him to end an ongoing Flynn investigation, his word will pack a punch.
The White House better batten down the hatches. A storm is brewing.
Why was Flynn being investigated?
Mr Trump’s early presidency has been overshadowed by a string of controversies regarding possible links to Russian interests.
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 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 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.
Contacts between Russia and members of the president’s campaign team are also under scrutiny.
Mr Comey’s sacking on 9 May fuelled claims that Mr Trump may have been trying to cover up his associates’ Moscow ties.
What is the reaction?
Both Democratic and Republican lawmakers were swift to question Mr Comey’s memo following the report.
Republican Jason Chaffetz, who chairs the House Oversight Committee, tweeted the panel would “get the Comey memo, if it exists. I need to see it sooner rather than later. I have my subpoena pen ready”.
This demand by Mr Chaffetz for the memo was backed by House Speaker Paul Ryan, although Mr Ryan’s deputy Kevin McCarthy said there were a “a lot of allegations and… small truth”.
Top Democratic Senator Chuck Schumer said he was “shaken” by the New York Times report.
“The country is being tested in unprecedented ways. I say to all of my colleagues in the Senate, history is watching,” he said.
And Adam Schiff, the highest ranked Democrat on the House Intelligence Committee, said this amounted to “interference or obstruction of the investigation”.