"James Comey better hope that there are no 'tapes' of our conversations before he starts leaking to the press!" Trump said in one of a string of Twitter messages.
The warning appeared to suggest that if Comey gave his version of events, the administration might produce tapes of conversations to counter that, although it was not clear if such tapes exist.
His veiled threat was likely to add to the storm roiling Washington over Trump's abrupt firing of Comey on Tuesday.
Critics have assailed Trump for firing the FBI chief at a time when the agency is investigating alleged Russian meddling in the 2016 U.S. election, and possible collusion between Moscow and the Trump presidential campaign.
Trump said in his brief statement about firing Comey on Tuesday that the FBI chief had told him three times that he was not under investigation in the Russia probe. In an interview on Thursday with NBC News, the president said Comey gave him this assurance during a dinner and in two phone conversations.
Comey has not publicly discussed any conversations he has had with Trump.
The man who took over as acting FBI director, Andrew McCabe, sidestepped a question during a Senate hearing on Thursday on whether he ever heard Comey tell Trump that the president was not the subject of investigation.
The FBI probe and parallel congressional investigations have hung over Trump's presidency since he took office in January and threaten to overwhelm his policy priorities.
Russian President Vladimir Putin ordered a campaign of interference in the election, U.S. intelligence agencies concluded in January, aimed at tilting the vote in Trump's favor. Moscow has denied interference in the election, and the Trump administration denies allegations of collusion with Russia.
'PRESIDENT TRUMP IS DANGEROUS'
The New York Times reported on Friday that Comey had resisted a request from Trump that he pledge loyalty to the president. The ousted director has told associates he was summoned to the White House for a one-on-one dinner with Trump just seven days after the Jan. 20 inauguration, according to the paper.
During the dinner, the president asked the FBI director to pledge loyalty to him, the Times reported. Comey declined to make that promise but told the president he could count on his honesty, the paper said.
According to Comey's account to associates, Trump was not satisfied with the FBI director's answer and told him again later in the dinner that he needed Comey's loyalty, the Times reported.
Comey again promised his "honesty," but did not pledge loyalty, the Times reported.
The U.S. Senate's No. 2 Democrat, Richard Durbin, slammed the president after Friday's tweet about Comey.
"I think we ought to get to the bottom line here: President Trump is dangerous," Durbin told MSNBC, the cable news channel. "He's dangerous because he may be obstructing justice in terms of the investigation ... and secondly his credibility has been destroyed."
In a series of tweets on Friday, Trump also responded to media reports questioning the credibility of White House statements on the Comey firing, which have changed over the course of the week.
"As a very active President with lots of things happening, it is not possible for my surrogates to stand at podium with perfect accuracy!" Trump said.
"Maybe the best thing to do would be to cancel all future 'press briefings' and hand out written responses for the sake of accuracy???"
The White House has said Comey's firing was unrelated to the Russia probe, but in the interview with NBC News on Thursday, Trump said he knew he ran the risk that by firing Comey he would "confuse people" and "lengthen out the investigation" into ties to Russia.
"In fact 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, it's an excuse by the Democrats for having lost an election that they should have won," the Republican president told NBC.
He said he never pressured Comey into dropping the FBI probe, adding: "If Russia did anything, I want to know that." Trump said there was no "collusion between me and my campaign and the Russians," and that "the Russians did not affect the vote."
The White House initially said Comey was fired on the recommendation of the two top officials at the Justice Department. On Thursday Trump said he would have fired Comey regardless of any such recommendations.