WASHINGTON (BLOOMBERG) - US President Donald Trump resumed a full-throated attack on Mr James Comey, hours before the FBI director he fired in May will appear in a network television interview ahead of the release of a book that describes the president in unfavourable terms.

"I never asked Comey for Personal Loyalty. I hardly even knew this guy. Just another of his many lies. His 'memos' are self serving and FAKE!" Mr Trump said on Twitter early Sunday (Apr 15), one of five Comey-related tweets of the morning.

"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!" Mr Trump also said in what so far has been the final tweet.

Mr Trump's tweet storm comes ahead of a highly anticipated interview with Mr Comey on ABC News to be broadcast on Sunday night. Mr Comey will discuss his memoir, "A Higher Loyalty: Truth, Lies and Leadership", which is scheduled for release on Tuesday.

Mr Comey, who ran the Federal Bureau of Investigations for almost four years, portrays the president in the book as a liar, dumb and rude, and he compares elements of Mr Trump's behaviour to that of a Mafia boss.

The thought of having dinner alone with Mr Trump at the White House made Mr Comey "deeply uncomfortable", he writes in the book, because he feared that the president wanted to compromise the FBI's integrity and independence and was waging "an effort to establish a patronage relationship" with him.

The Republican National Committee has started an effort to discredit Mr Comey on social media, in digital advertising, and through a website, lyincomey.com, ahead of the release of the book.

White House press secretary Sarah Huckabee Sanders said last Friday that Mr Comey has a "credibility problem" and that firing him will go down as one of Mr Trump's greatest achievements.

On ABC's "This Week", she said it was clear that Mr Comey was a "leaker" by his own admission and "there is such a huge bipartisan consensus that James Comey doesn't have credibility and shouldn't have been leading the FBI any longer".

House Speaker Paul Ryan said on Sunday that Mr Comey is a man of integrity "as far as I know", while adding that "I don't know the guy".

Asked whether Mr Comey is someone he would describe as a slime ball - a term Mr Trump has used twice in tweets this week - Mr Ryan said in an interview broadcast on Sunday on NBC's "Meet the Press" that "I don't speak like that. I don't use words like that".