Hours after President Donald Trump claimed via Twitter that he never spoke to author Michael Wolff for the shocking exposé, Fire and Fury: Inside the Trump White House, Wolff said otherwise.

"I absolutely spoke to the president… it certainly was not off the record," Wolff told Savannah Guthrie on NBC's Today.

Late Thursday, Trump tweeted about Fire and Fury, excerpts from which exploded online this week. "I authorized Zero access to White House (actually turned him down many times) for author of phony book! I never spoke to him for book. Full of lies, misrepresentations and sources that don't exist. Look at this guy's past and watch what happens to him and Sloppy Steve!" Trump wrote, coining a new nickname for ousted White House chief strategist Steve Bannon in the process.

Wolff's book includes numerous embarrassing personal allegations about the president, including that daughter Ivanka mocked the president's hair and that Trump allegedly chastised White House staff for picking up his dirty laundry off the floor. Fire and Fury also quotes Bannon as saying Trump's son Donald Jr. was "unpatriotic" and "treasonous" for meeting with a Kremlin-connected lawyer promising compromising details about Hillary Clinton during the 2016 election campaign.

Asked Friday about his sourcing and reporting, Wolff maintained that his work was credible. "I work like every journalist works, so I have recordings, I have notes," he told Guthrie. "I am certainly, in absolutely every way, comfortable with everything I've reported in this book."

In addition to his tweet, Trump had his lawyers send a cease-and-desist letter to publisher Henry Holt and Co. demanding the release of Fire and Fury be stopped. "Mr. Trump hereby demands that you immediately cease and desist from any further publication, release or dissemination of the book, the article, or any excerpts or summaries of either of them, to any person or entity, and that you issue a full and complete retraction and apology to my client as to all statements made about him in the book and article that lack competent evidentiary support," the letter said, per the New York Times.

In response, Henry Holt moved the book's publication up to Friday. "We see Fire and Fury as an extraordinary contribution to our national discourse, and are proceeding with the publication of the book," the publisher said in a statement.

Asked about Trump's tactics, Wolff told Guthrie he was pleased with the attention. "Not only is he helping me sell books, but he's helping me prove the point of the book," he said. "I mean, this is extraordinary that a President of the United States would try to stop the publication of a book. This doesn't happen. This has not happened from other presidents, would not even happen from a CEO of a mid-size company." He later added, "I think one of the things we have to count on is that Donald Trump will attack, he will send lawyers' letters — this is a 35-year history of how he approaches everything."

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