Former FBI director James Comey has said it is possible that Russia may have compromising information on US president Donald Trump.
Mr Comey added that that there is "some evidence of obstruction of justice" in the president's actions, and that Mr Trump is "morally unfit" for office.
The ex-FBI chief's comments in an ABC News interview are almost certain to escalate his war of words with the president.
Mr Trump's adviser Kellyanne Conway accused Mr Comey of peddling a "revisionist version of history" and sinking into the "gutter" with petty comments about the size of Mr Trump's hands and the length of his tie.
"He looked a little shaky," Ms Conway said of Mr Comey, on ABC's Good Morning America.
Hours before the interview was shown, the president, who sacked Mr Comey last year, unleashed a Twitter outburst that labelled him "slippery", suggested he should be put in jail and branded him "the WORST FBI Director in history, by far!"
Slippery James Comey, a man who always ends up badly and out of whack (he is not smart!), will go down as the WORST FBI Director in history, by far!— Donald J. Trump (@realDonaldTrump) April 15, 2018
Mr Comey's televised remarks, coupled with the release of his forthcoming book, offer his version of events surrounding his sacking and the investigations into Russian election meddling and Hillary Clinton's email practices.
Several of the episodes he describes in detail, including a private conversation about former White House national security adviser Michael Flynn, are central to special counsel Robert Mueller's investigation into Russian interference and his recollections are presumably valuable for prosecutors examining whether the president's actions constitute an obstruction of justice.
The FBI director, who until his sacking last May led an investigation into possible ties between Russia and the Trump campaign, acknowledged that it was "stunning" to think that Russia could hold damaging information about an American president.
But he said that in Mr Trump's case, he could not discount the possibility that the president had been compromised.
"These are more words I never thought I'd utter about a president of the United States, but it's possible," Mr Comey told ABC News.
.@GStephanopoulos: “Do you think the Russians have something on Donald Trump?”
@Comey: “I think it's possible. I don't know. These are more words I never thought I'd utter about a president of the United States, but it's possible.” https://t.co/itnTAb22eu #Comey pic.twitter.com/MN7YOFKDkf— ABC News (@ABC) April 16, 2018
He also answered "possibly" when asked if the president was attempting to obstruct justice when he cleared the Oval Office of other officials last February before encouraging him to close the investigation into Mr Flynn, who by that point was suspected of lying to the FBI about his Russian contacts. The retired general pleaded guilty last December and is now co-operating with Mr Mueller's investigation.
Mr Comey also said he believed that Mr Trump was "morally unfit" to be president and that he treated women like "pieces of meat".
"A person who sees moral equivalence in Charlottesville, who talks about and treats women like they're pieces of meat, who lies constantly about matters big and small and insists the American people believe it - that person's not fit to be president of the United States, on moral grounds," he added.
Mr Trump rejected Mr Comey's assertion that the president had sought his loyalty at a January 2017 dinner, saying: "I hardly even knew this guy. Just another of his many lies."
- Digital Desk and Press Association