The stunning pronouncement gave more ammunition to critics who say Trump fired Comey to thwart his bureau's investigation into whether Trump associates colluded with Russian Federation in the 2016 election.
In an early-morning tweetstorm Friday, Trump suggested there might be recordings of his conversations with Comey that could be used to undercut anything the former director might tell the public.
"And in fact, when I chose to just do it, I said to myself, 'You know, this Russian Federation thing with Trump and Russian Federation is a made up story, ' " he said. "The integrity of our law enforcement agencies and their leadership is of the utmost importance to President Trump".
"James Comey better hope that there are no "tapes" of our conversations before he starts leaking to the press!"
"I'm not aware of that", Spicer said of a potential tape.
Mr Comey has not publicly discussed any conversations he had with Trump.
The Federal Bureau of Investigation probe and parallel congressional investigations have cast a cloud over Trump's presidency since he took office on January 20, threatening to overwhelm his policy priorities.
The White House has said Comey's firing was unrelated to the Russian Federation 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 Russian Federation.
The U.S. Senate's No. 2 Democrat, Richard Durbin, slammed the president after Friday's tweet about Comey.
Mr Trump also said this week that he alone was responsible for the decision to sack Mr Comey, calling him a "showboat" and "grandstander".
Shortly after the interview was published, Jonathan Karl, the chief White House correspondent for ABC News, asked Sanders if she, Spicer and Pence had been "left in the dark" about Trump's decision making or had been misled.More news: Ebola-Zaire confirmed in DRC outbreak that killed 3
More news: Aisha Buhari meets 82 Chibok school girls in Abuja
More news: Wizards' Oubre: Olynyk hits led to outburst
In a series of tweets, Mr Trump threatened Mr Comey and also warned that he may cancel all White House press briefings. "Instead, Mr. Comey has recounted to others, he told Mr. Trump that he would always be honest with him, but that he was not "reliable" in the conventional political sense".
"As a very active President with lots of things happening, it is not possible for my surrogates to stand at podium with ideal accuracy!" he said.
"He wanted to stay on", Trump said.
Spicer pointed out that Clapper was director of national intelligence up until January 20, suggesting Clapper would have known if collusion had occurred.
"When James Clapper himself, and virtually everyone else with knowledge of the witch hunt, says there is no collusion, when does it end?"
Sanders insisted that Comey was terminated because he mishandled the investigation into Clinton's email server - an inquiry that Trump cheered during the campaign.
President Donald Trump on Friday ratcheted-up the rhetoric against fired FBI Director James Comey, issuing a warning via Twitter to Comey that he shouldn't leak information to the media. "As a very active President with lots of things happening, it is not possible for my surrogates to stand at podium with ideal accuracy!" he tweeted.
Trump fired Comey on Tuesday, and the president said in an NBC News exclusive interview on Thursday that he had been planning to do so regardless of what the Department of Justice recommended.
Even before Mr Trump's provocative tweet, the White House was scrambling to clarify why Mr Comey was sacked. "It really places a burden on the White House to be more forthcoming to provide a better explanation as to the timing".
The president said he never pressured Comey into dropping the Federal Bureau of Investigation probe, and added that there was no "collusion between me and my campaign and the Russians".