Donald Trump's chief-of-staff, General John Kelly, has repeatedly referred to the US president as an "idiot", it has been claimed.
The assertion is made in the latest book on the inner workings of the Trump administration, which paints an unflattering picture of the president's relationship with members of his staff.
It is also said that aides have removed papers from Mr Trump's desk to prevent him signing them.
Fear: Trump In The White House, by veteran reporter Bob Woodward, claims Gen Kelly has repeatedly called Mr Trump an "idiot" and quotes him as saying "it's pointless to try to convince him [Mr Trump] of anything".
The former marine general is also said to have doubted Mr Trump's mental faculties, declaring during one meeting: "We're in crazytown."
Defence secretary James Mattis apparently told a staffer the president had the foreign policy understanding of a "fifth- or sixth-grader" - a child of 11 or 12.
The Woodward book has already been refuted and discredited by General (Secretary of Defense) James Mattis and General (Chief of Staff) John Kelly. Their quotes were made up frauds, a con on the public. Likewise other stories and quotes. Woodward is a Dem operative? Notice timing?— Donald J. Trump (@realDonaldTrump) September 4, 2018
Mr Trump responded on Twitter by posting statements credited to Mr Mattis, Gen Kelly and from the White House press secretary Sarah Sanders.
He suggested the book was published at a time to boost the Democrats during the Supreme Court power struggle, writing: "The Woodward book has already been refuted and discredited by General (Secretary of Defense) James Mattis and General (Chief of Staff) John Kelly.
"Their quotes were made up frauds, a con on the public. Likewise other stories and quotes. Woodward is a Dem operative? Notice timing?"
He added: "The already discredited Woodward book, so many lies and phony sources, has me calling Jeff Sessions 'mentally retarded' and 'a dumb southerner.'
"I said NEITHER, never used those terms on anyone, including Jeff, and being a southerner is a GREAT thing."
The book claims that Gary Cohn, the former director of the national economic council, boasted of removing papers from the president's desk to prevent him withdrawing from the North American Free Trade Agreement (NAFTA).
Mr Trump's former lawyer in the Russia inquiry, John Dowd, is said to have told Mr Trump not to agree to be interviewed by special counsel Robert Mueller as he doubted his boss's ability to avoid perjuring himself.
"Don't testify. It's either that or an orange jumpsuit," Mr Dowd is said to have told the president.
Mr Mattis is quoted explaining to Mr Trump why the US maintains troops on the South Korean peninsula to monitor the North's missile activities.
"We're doing this in order to prevent World War III," Mr Mattis said.
General Kelly has denied the book's claims, saying in a statement: "The idea I ever called the president an idiot is not true... This is another pathetic attempt to smear people close to President Trump and distract from the administration's many successes."
White House press secretary Sarah Sanders called the book "nothing more than fabricated stories, many by former disgruntled employees".
Mr Woodward, who along with Carl Bernstein helped expose the Watergate scandal that brought down President Richard Nixon in 1974, relied on so-called "deep background" conversations with sources, often recorded.
Current and former White House officials estimate that nearly all of their colleagues co-operated with Mr Woodward.
However, his sourcing was nonetheless questioned by Mr Trump in a telephone conversation in August, released by the Washington Post.
In the call, recorded by Mr Woodward with Mr Trump's permission, the president first says he did not know about the book, then changes tack and admits he did, blaming staff for preventing the reporter from reaching him to discuss it.
The latest book is the third this year to lift the lid on Mr Trump's White House, following Fire And Fury by Michael Wolff and an expose of her time in the West Wing by former White House aide and Apprentice contestant Omarosa Manigault Newman.
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