US President Donald Trump unleashed a spectacular denunciation of one of his closest political allies on Wednesday, describing former White House chief strategist Steve Bannon as insane and irrelevant.

After the release of explosive excerpts from a new book in which Bannon reportedly described Trump's eldest son's meeting with a Kremlin-connected lawyer as "treasonous" and "unpatriotic," Trump wasted no time firing back.

The embattled Republican president issued a response that was searing even by his combative standards.

"Steve Bannon has nothing to do with me or my presidency. When he was fired, he not only lost his job, he lost his mind," Trump said in a written statement.

Trump said Bannon — who engineered the New York real estate mogul's link to the nationalist far right and helped create a pro-Trump media ecosystem — was "only in it for himself."

Trump's fury appeared to have been provoked by the publication of startling extracts from "Fire and Fury: Inside the Trump White House" by author Michael Wolff.

Passages published by The Guardian and New York magazine quoted Bannon being highly critical of Trump's son Donald Junior and daughter Ivanka.

Bannon, who left the White House in August, is also quoted as saying that the investigation by special counsel Robert Mueller into Russian interference in the 2016 election will focus on money laundering.

The investigation by Mueller, a former FBI director, is looking into whether the Trump campaign colluded with Russia to get him elected — a charge the president has repeatedly and vehemently denied.

US media reported on Wednesday that a lawyer representing Trump has sent a cease-and-desist letter accusing his former chief strategist of violating a non-disclosure agreement by speaking to the author of the upcoming book.

In a letter to Bannon quoted by US media, attorney Charles Harder wrote: “You have breached the Agreement by, among other things, communicating with author Michael Wolff about Mr. Trump, his family members” and his presidential campaign.

'You should have called the FBI'

Donald Junior took a meeting with Russian lawyer Natalia Veselnitskaya in June 2016 after an intermediary promised material that would incriminate Trump's Democratic rival Hillary Clinton.

Trump's son-in-law Jared Kushner and then campaign chairman Paul Manafort also attended the meeting at Trump Tower in New York.

"The three senior guys in the campaign thought it was a good idea to meet with a foreign government inside Trump Tower in the conference room on the 25th floor — with no lawyers," Bannon was quoted as saying in the book.

"They didn't have any lawyers.

"Even if you thought that this was not treasonous, or unpatriotic, or bad shit, and I happen to think it's all of that, you should have called the FBI immediately," he said.

'Little to do' with election win

Trump responded quickly and cuttingly to the reported comments by Bannon, a former investment banker and the executive chairman of influential ultraconservative outlet Breitbart News.

"Now that he is on his own, Steve is learning that winning isn't as easy as I make it look. Steve had very little to do with our historic victory, which was delivered by the forgotten men and women of this country," Trump said.

"Steve pretends to be at war with the media, which he calls the opposition party, yet he spent his time at the White House leaking false information to the media to make himself seem far more important than he was," he added.

"Steve was rarely in a one-on-one meeting with me and only pretends to have had influence to fool a few people with no access and no clue, whom he helped write phony books."

Disbelief over victory -

Wolff's book — which he says is based on interviews with Trump, his senior aides and others — also mentions that Trump did not initially know who former House speaker John Boehner was, that he eats food from McDonald's because he believes it to be safe from poison, and that his team did not believe he was capable of winning the election.

"Shortly after 8 pm on Election Night, when the unexpected trend — Trump might actually win — seemed confirmed, Don Jr. told a friend that his father, or DJT, as he calls him, looked as if he had seen a ghost. Melania was in tears — and not of joy," it said.

The book even deals with the subject of Trump's infamous coiffe, citing Ivanka as telling friends it was the result of a comb-over from the front and sides of his head, stiffened by hair spray.

"The color, she would point out to comical effect, was from a product called Just for Men — the longer it was left on, the darker it got. Impatience resulted in Trump's orange-blond hair color."

Trump's press secretary Sarah Sanders hit back.

"This book is filled with false and misleading accounts from individuals who have no access or influence with the White House," she told a briefing.

Stephanie Grisham, a spokeswoman for First Lady Melania Trump, denied allegations that she had cried tears of sadness.

"The book is clearly going to be sold in the bargain fiction section. Mrs. Trump supported her husband's decision to run for President and in fact, encouraged him to do so. She was confident he would win and was very happy when he did," Grisham said in a statement.