"He was insane, a real nut job", Trump said, according to the New York Times.
The details of his comments to the Russians would seem to bolster theories that Trump fired Comey in an effort to choke off the Russia investigation.
This last part of the deputy attorney general's account seems to contradict the White House's explanation of how Trump made a decision to fire Comey.
The reports, emerging just as Trump jetted off to Saudi Arabia on his first foreign trip as president, were likely to extend the turmoil engulfing his administration since the May 9 firing of Comey.
The topics, including the Justice Department's decision to appoint Former FBI Director Robert Mueller as Special Counsel to investigate Russian meddling in the 2016 election, overshadowed President Trump's third visit of the week from a foreign leader.
It quoted unnamed sources familiar with the investigation as saying it was someone "close to the president" but they would not identify them further.
The White House has been thrown into turmoil by a string of allegations against Mr Trump this week, including that he may have obstructed justice by asking Mr Comey to drop an investigation into one of his top advisors.
The Times report added to the impression, given by Trump himself in a television interview last week, that the Russian Federation issue was a factor in firing Comey.
Toobin believes people are focusing on the wrong part of Trump's reported remarks - as much of the public's attention has been devoted to Trump reportedly calling Comey a "nut job".
"This new report that President Trump openly admitted to the Russians that he "faced great pressure" from the FBI's criminal investigation that was "taken off" when he fired Director Comey is astonishing - and extremely troubling", Cummings said.
The New York Times reported on Friday that Trump discussed the firing of Comey with Russian Foreign Minister Sergei Lavrov when Moscow's top diplomat visited the White House the day after the Federal Bureau of Investigation chief was dismissed.
The memo is said to have been written by Mr Comey the day after Mr Flynn resigned.
Trump, accompanied by wife Melania, walk across the South Lawn of the White House in Washington on Friday.
Comey has agreed to testify before the Senate intelligence committee, although a date has not yet been set, according to the committee's chairman, Senator Richard Burr.
As special counsel, Mueller is "authorized to prosecute federal crimes arising from the investigation of these matters", according to the Justice Department order Rosenstein signed.
Investigators into Russian meddling in the U.S. presidential elections are now also probing whether White House officials have engaged in a cover-up, according to members of Congress who were briefed Friday by Deputy Attorney General Rod Rosenstein.
Trump, at a combative and complaining press conference Thursday, he fell short in trying to resolve questions about investigations into his campaign and his first four months in office.
Comey turned down an invitation to testify before the Senate Judiciary Committee.
Days after he won the United States presidential election last November, Donald Trump announced the nomination of Sessions as attorney general. It immediately escalated the legal stakes - and the potential political damage - for a president who has tried to dismiss the matter as partisan witch hunt and a "hoax".
Trump calls appointment of special counsel a 'witch hunt'
If Mueller wraps up the probe relatively quickly and finds no damning evidence, that would also be a boon for the White House. Bush's administration leaked the name of Central Intelligence Agency operative Valerie Plame to reporters.
Thomas injured, Celtics find new low in Game 2 loss to Cavs
Cleveland is just playing too well right now and an NBA Finals matchup with the Warriors is inevitable. "The game's over. Thomas will have additional tests Saturday and will be re-evaluated before the team determines his status for Game 3.
Trump brags to Russians about firing 'nut job' Comey
He added, "The investigation would have always continued, and obviously, the termination of Comey would not have ended it". The Justice Department on Friday issued the text of Rosenstein's opening remarks for the briefings on Capitol Hill.