A new book by a reporter who helped bring down former US president Richard Nixon is roiling the White House, with current and former aides of President Donald Trump quoted as calling him an "idiot" and admitting they snatched sensitive documents off his desk to keep him from taking rash actions.

The book by Washington Post journalist Bob Woodward is the latest to throw the Trump administration into damage-control with explosive anecdotes and concerns about the commander in chief.

The Post on Tuesday (local time) published details from Fear: Trump in the White House, the Watergate reporter's forthcoming examination of Mr Trump's first 18 months in office.

Publication of Woodward's book has been anticipated for weeks, and current and former White House officials estimate that nearly all their colleagues cooperated with the famed Watergate journalist.

White House chief of staff John Kelly allegedly called Mr Trump an "idiot". ( AP: Pablo Martinez Monsivais )

The White House, in a statement from press secretary Sarah Sanders, dismissed the book as "nothing more than fabricated stories, many by former disgruntled employees, told to make the President look bad".

The book says chief of staff John Kelly had doubts about Mr Trump's mental faculties, and declared during one meeting, "we're in Crazytown".

It also says he called Mr Trump an "idiot," an account that Mr Kelly denied.

"The idea I ever called the President an idiot is not true," Mr Kelly said in a statement.

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The book says that Mr Trump's former lawyer in the Russia probe, John Dowd, doubted the President's ability to avoid perjuring himself should he be interviewed in special counsel Robert Mueller's investigation into Russian election interference and potential coordination with Mr Trump's campaign. Mr Dowd stepped down in January.

"Don't testify. It's either that or an orange jumpsuit," Mr Dowd is quoted telling the President.

Mr Dowd, in a statement, said "no so-called 'practice session' or 're-enactment'" took place, and denied saying that Mr Trump was likely to end up in an orange jumpsuit.

Secretary of Defence Jim Mattis is quoted explaining to Mr Trump why the US maintains troops on the Korean Peninsula to monitor North Korea's missile activities.

"We're doing this in order to prevent World War III," Mr Mattis said, according to the book.

The book recounts that Mr Mattis told "close associates that the President acted like — and had the understanding of — 'a fifth- or sixth-grader'".

But in a statement, Mr Mattis said "the contemptuous words about the President attributed to me in Woodward's book were never uttered by me or in my presence".

"While I generally enjoy reading fiction, this is a uniquely Washington brand of literature, and his anonymous sources do not lend credibility."

Mr Mattis said the notion that he would "show contempt" for Mr Trump or "tolerate disrespect" to the office of the President of the United States "is a product of someone's rich imagination".

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Mattis ignored Trump's plea to 'kill' Syrian leader

Woodward reported that after Syria's Bashar Assad launched a chemical weapons attack on civilians in April 2017, Mr Trump called Mr Mattis and said he wanted the Syrian leader taken out, saying, "kill him! Let's go in".

Mr Mattis assured Mr Trump he would get right on it, but then told a senior aide they would do nothing of the kind, Woodward wrote. National security advisers instead developed options for the airstrike that Mr Trump ordered.

Mr Trump allegedly said Jeff Sessions was "mentally retarded". ( AP: Evan Vucci )

Woodward also claims that Gary Cohn, the former director of the National Economic Council, boasted of removing papers from the President's desk to prevent Mr Trump from signing them into law, including efforts to withdraw from the North American Free Trade Agreement.

The book also quotes Mr Trump as mocking his Attorney-General Jeff Sessions, who has been a target of the President's wrath since recusing himself from the Russia investigation.

"This guy is mentally retarded," Mr Trump said of Mr Sessions, according to the book.

"He couldn't even be a one-person country lawyer down in Alabama."

Mr Trump did not speak to Woodward until after the book's manuscript was completed.

The Post released audio of Mr Trump expressing surprise about the book in an August conversation with Woodward.

Sorry, this video has expired Woodward book paints picture of White House in chaos

Woodward tells Mr Trump he had contacted multiple officials to attempt to interview Mr Trump and was rebuffed.

The book shot to the top of the Amazon charts a week before its release. ( Simon & Schuster via AP )

The book follows the January release of author Michael Wolff's Fire and Fury, which led to a rift between Mr Trump and Steve Bannon, his former chief strategist, who spoke with Wolff in terms that were highly critical of the President and his family.

Wolff's book attracted attention with its vivid anecdotes but suffered from numerous factual inaccuracies.

Woodward's work also comes weeks after former White House aide and Apprentice reality show contestant Omarosa Manigault Newman published an expose on her time in the West Wing, including audio recordings of her firing by Mr Kelly and a follow-up conversation with the President in which he claimed to have been unaware of Mr Kelly's decision.

While White House aides have become increasingly numb to fresh scandals, the book still increased tensions in the West Wing, especially given the intimate details shared and the number of people Woodward appeared to have interviewed.

Some White House officials expressed surprise at the number of erstwhile Trump loyalists willing to offer embarrassing stories of the President and his inner circle.

No clear response strategy emerged immediately after the Post report, but insiders speculated the fallout could be worse than from Fire and Fury, given Woodward's storied reputation.

Journalist Bob Woodward was involved in the Watergate story that brought down Richard Nixon. ( AP: Alex Brandon )

Scheduled for a September 11 release, Fear was ranked the top-selling book on Amazon on Tuesday.

Mr Trump has been increasingly critical of anonymous sources used by reporters covering his administration.

Woodward's account relies on deep background conversations with sources, meaning their identities are not disclosed.

Former George W Bush press secretary Ari Fleischer defended Woodward's methodology.

"I've been on the receiving end of a Bob Woodward book," he tweeted.

"There were quotes in it I didn't like. But never once — never — did I think Woodward made it up," Mr Fleischer said.

"Anonymous sources have looser lips and may take liberties. But Woodward always plays is straight. Someone told it to him."

AP