The White House under President Donald Trump is mired in a perpetual "nervous breakdown" with staff constantly seeking to control a leader whose anger and paranoia can paralyze operations for days, according to a new book by Bob Woodward.

The Washington Post, which obtained an advance copy of the book by the veteran chronicler of modern presidents, reported Tuesday that Woodward describes Trump manically pressing his staff for actions that could lead to major conflict -- leaving them little choice but to disregard his orders.

In one anecdote, Trump questioned his National Security Council on January 19 about why the United States maintained a costly presence on the Korean peninsula.

"We're doing this in order to prevent World War III," Defense Secretary Jim Mattis felt it necessary to inform him.

After that meeting, according to Woodward, Mattis said to colleagues that Trump had the understanding "a fifth- or sixth-grader" -- a 10- or 11-year-old child.

Woodward's much-awaited book, "Fear: Trump in the White House," shows aides routinely ignoring or acting to block orders by the US president.

Gary Cohn, who was the chief White House economic advisor, went so far last year as to steal an order from Trump's desk that, if the president had signed it, would have cancelled the US-South Korea trade agreement.

And in April 2017, after Syria's Bashar al-Assad attacked civilians with chemical weapons, Woodward reports, Trump demanded Mattis assassinate Assad.

"Let's fucking kill him! Let's go in. Let's kill the fucking lot of them," Trump told the Pentagon chief.

Mattis agreed to take action, but after he got off the telephone, he told an aide that the United States would take "more measured" steps against Syria -- which eventually came as a punitive air strike.

According to Woodward, Chief of staff John Kelly told colleagues he saw Trump as "unhinged" and "an idiot."

"It's pointless to try to convince him of anything. He's gone off the rails. We're in Crazytown," Kelly said, according to Woodward.

The writer, one of a duo with Carl Bernstein whose reporting for the Post on the Watergate scandal brought down president Richard Nixon, has published powerful and often embarrassing insider books on presidents George W Bush and Barack Obama.

The book is scheduled for official launch on September 11.