US President Donald Trump wanted to have Syrian President Bashar al-Assad assassinated last year but his defence secretary ignored the request, according to a new book by Watergate reporter Bob Woodward.

Excerpts from the book, entitled 'Fear' were published by The Washington Post.

The book, which has not yet been released, depicts top Trump aides sometimes sidestepping instructions to limit what they see as his damaging and dangerous behavior.

It is the latest to detail tension within the White House under Mr Trump's 20-month-old presidency.

The book portrays Mr Trump as prone to profane outbursts and impulsive decision-making, painting a picture of chaos that Mr Woodward says amounts to an "administrative coup d’etat" and a "nervous breakdown" of the executive branch.

According to the book, Mr Trump told Defense Secretary Jim Mattis that he wanted to have Mr Assad assassinated after the Syrian president launched a chemical attack on civilians in April 2017.

Mr Mattis told Mr Trump he would "get right on it," but instead developed a plan for a limited air strike that did not threaten Mr Assad personally.

Mr Mattis told associates after a separate incident that Mr Trump acted like "a fifth or sixth-grader (primary school student)," according to the book.

Tonight, the White House said the excerpts were a fabrication by disgruntled former staff members.

Mr Woodward, who gained fame for his reporting on the Watergate scandal in the 1970s, talked to top aides for the book with the understanding that he would not reveal how he got his information, The Washington Post said.

Among his other revelations: former top economic adviser Gary Cohn stole a letter off Mr Trump's desk that the president planned to sign that would withdraw the United States from a trade agreement with South Korea.

Mr Cohn, who tried to rein in Mr Trump's protectionist impulses, also planned to remove a similar memo that would have withdrawn the United States from the North American Free Trade Agreement with Mexico and Canada, Mr Woodward wrote.

"I'll just take the paper off his desk," Mr Cohn told another White House aide, according to the book.

The United States remains part of both trade agreements as it negotiates new terms.

Other aides reportedly insulted Mr Trump behind his back. Chief of Staff John Kelly allegedly called Mr Trump an "idiot," and said: "We're in Crazytown ... This is the worst job I've ever had."

However, Mr Kelly has denied making the comments.

Mr Trump treated top aides with scorn, the book claims, telling Commerce Secretary Wilbur Ross that he was past his prime and calling Attorney General Jeff Sessions "mentally retarded".

Mr Trump has grown paranoid and anxious over an ongoing federal inquiry into whether his campaign colluded with Russia in Moscow's alleged interference in the 2016 presidential election, prompting aides to compare him to former President Richard Nixon during the Watergate scandal, Mr Woodward reported.

Mr Trump's former lawyer, John Dowd, conducted a mock interview with Mr Trump to convince him that he would commit perjury if he agreed to talk to Special Counsel Robert Mueller, who is leading the Russia investigation, the book claims.

Mr Trump did not speak to him until the manuscript was complete, the paper said.

"So I have another bad book coming out. Big deal," Mr Trump told Mr Woodward, according to a transcript of a telephone call released by The Washington Post.