President Donald Trump has launched a verbal tirade against James Comey after the former FBI director described him as an unethical liar in an explosive new memoir.
The president fired Comey last May while his agency was investigating potential collusion between Trump's campaign and Russia in the 2016 US election in a move that led the Justice Department to appoint Special Counsel Robert Mueller to take over a probe that has hung over his presidency.
"This president is unethical, and untethered to truth and institutional values," Comey said in the book due out Tuesday.
Trump has often publicly criticised Comey since firing him, but escalated his attacks in response to the book.
"It was my great honour to fire James Comey!" Trump said in one of a series of scorching Twitter messages, adding that Comey - now one of the Republican president's fiercest critics - had been a terrible FBI director.
The tirade followed news accounts of Comey's book, "A Higher Loyalty: Truth, Lies and Leadership," which paints a deeply unflattering picture of Trump, comparing him to a mob boss who stresses personal loyalty over the law and has little regard for morality or truth.
Mueller is looking into whether Trump has sought to obstruct the Russia probe, and Comey could be a key witness on that front.
Comey last year accused Trump of pressuring him to pledge loyalty and end a probe involving former national security adviser Michael Flynn's contacts with Moscow.
"James Comey is a proven LEAKER & LIAR," Trump wrote.
Trump accused Comey of lying to Congress, but did not specify was he was referring to, and said the former FBI chief should be prosecuted for leaking classified information.
Trump has denied any collusion and has called Mueller's investigation a witch hunt.
Comey is conducting a series of media interviews before the book's official release.
The interviews are Comey's first public comments since he testified before the Senate Intelligence Committee last June, when he accused Trump of firing him to undermine the FBI's Russia investigation.
Just days after Trump fired Comey, the president said he did it because of "this Russia thing."
Trump has launched a series of attacks since last year against US law enforcement leaders and institutions as the Russia probe pressed forward, in addition to Comey and Mueller.
"People will rot in hell for besmirching the reputation the integrity and the professional history of these two men," Democratic US Representative Jim Himes said on CNN, referring to Comey and Mueller, himself a former FBI director.