Washington: The author of a book critical of Donald Trump's first year in office insists he had spoken with the President, contradicting Trump's assertion that he had never talked to the writer for the book and had authorised "Zero access" to the White House.

Michael Wolff's book Fire and Fury: Inside the Trump White House went on sale on Friday, its release pulled forward after published excerpts this week set off a political firestorm, threats of legal action and an effort to halt publication.

The book depicts a chaotic White House, a president who was ill-prepared to win the office in 2016, and Trump aides who scorned his abilities.

"I absolutely spoke to the President. Whether he realised it was an interview or not, I don't know, but it was certainly not off the record," Wolff said on NBC's Today program, adding he had spoken to people who spoke to Trump on a daily, "sometimes minute-by-minute" basis.