"James Comey better hope that there are no 'tapes' of our conversations before he starts leaking to the press!" Trump wrote in one of a series of early morning tweets assailing critics and the media over their response to Comey's ouster.
Trump asked his now-fired FBI director on three occasions whether he was the target of ongoing investigations, he said Thursday, stoking allegations of presidential interference.
WATCH: Trump offers explanation for Trump sacking
The US president on Friday acknowledged that Russia was on his mind when he made the decision to sack James Comey, who had been heading a probe into suspected Russian influence in the 2016 election.
"When I decided to just do it, I said to myself, I said you know, this Russia thing with Trump and Russia is a made up story," he said in an interview with NBC, appearing to link Comey's firing with the investigation.
Meanwhile, the New York Times reported that Trump pressed Comey for a pledge of loyalty over dinner only a week after his inauguration, according to an account by two associates of the lawman.
Comey, they said, declined to make such a pledge but told Trump he would always give him "honesty."
The Times said White House spokeswoman Sarah Huckabee Sanders disputed the account, saying Trump would "never even suggest the expectation of personal loyalty, only loyalty to our country and its great people."
The Times said it was not clear whether the dinner was the same one that Trump described in the NBC interview in which the president acknowledged asking Comey whether he was the subject of a counter-intelligence probe.
"I actually asked him, yes. I said, 'If it's possible would you let me know, am I under investigation?'"
"He said, 'You are not under investigation,'" Trump recounted, repeating an assertion made when the White House announced Comey's firing Tuesday.
"All I can tell you is, well I know what, I know that I'm not under investigation. Me. Personally. I'm not talking about campaigns. I'm not talking about anything else. I'm not under investigation."
The other two times Trump said he asked Comey whether he was under investigation were in telephone conversations.
Trump's comments to NBC raised questions about whether he had acted inappropriately and whether Comey had broken government guidelines in assuring him he was not under investigation.
US presidents are normally at pains to avoid any suggestion of interference or even commenting on ongoing investigations. The FBI typically does not confirm their existence.
Noted legal scholar Laurence Tribe told AFP that if Comey did indeed answer Trump's question, it would violate Department of Justice rules and "would be unthinkably unethical and unprofessional in this situation."
Trump also said that at the dinner, he and Comey discussed whether the US top cop would stay in his role and continue his ten-year term.
Asking such a question "would come close to bribery... or at least obstruction of justice, which Comey would've had to be an idiot to fall for by offering the assurance sought," Tribe said.
The White House rejected the suggestion that the exchange was inappropriate. "I don't see it as a conflict of interest," Sanders said.
In the interview, Trump insisted that he always intended to fire Comey, undercutting the initial White House explanation that he acted on the recommendation of top justice officials who criticized the FBI chief's handling of a probe into Hillary Clinton's emails.
"I was going to fire him regardless of recommendations," said the president. "He's a showboat, he's a grandstander."
What's the reason?
Opponents have claimed that Comey's shock sacking was a bid to stall an FBI investigation into alleged Russian meddling in the 2016 election, which is also looking into possible collusion between the Kremlin and Trump's team.
Trump's interview was released on the same day the interim head of the FBI told the US Congress that Comey's sacking would not derail the Russia investigation.
"There has been no effort to impede our investigation to date," acting director Andrew McCabe told the Senate Intelligence Committee, in a hearing that capped two days of high drama provoked by the dismissal.
"You cannot stop the men and women of the FBI from doing the right thing," he said. "All of the agents involved in the investigation are still in their positions."
While Trump said the outgoing FBI director had left the agency in "turmoil," McCabe rejected any suggestion that Comey had lost the support of the rank and file.
"Director Comey enjoyed broad support within the FBI and still does today," he told the committee.
Echoing a widely-held view in opposition ranks, the senior Democrat on the Senate committee, John Warner, called the timing of Comey's dismissal "especially troubling."
"He was leading an active counterintelligence investigation into any links between the Trump campaign and the Russian government or its representatives, and whether there was any coordination between the campaign and Russia's efforts to interfere in our election."
"For many people, including myself, it is hard to avoid the conclusion that the president's decision to remove Director Comey was related to this investigation."
McCabe gave no information on the status of the FBI probe, which dates back to July last year when the agency became aware of alleged Russian hacking of Democratic party computers and communications.
He also pledged not to discuss the Russia investigation with the White House.