WASHINGTON (AFP) - 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 paralyse 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 on Tuesday (Sept 4) that Mr Woodward describes Mr 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, Mr Trump questioned his National Security Council on Jan 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," Defence Secretary Jim Mattis felt it necessary to inform him.

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

Mr Woodward's much-awaited book, Fear: Trump In The White House, shows aides routinely ignoring or acting to block orders by the US President.

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

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

"Let's f***ing kill him! Let's go in. Let's kill the f***ing lot of them," Mr Trump told the Pentagon chief.

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

'CRAZYTOWN'

Mr Woodward, who does not name his sources but apparently had extensive access to many White House insiders, describes some of Mr Trump's top aides and Cabinet members as having deep disdain for him.

Chief of staff John Kelly told colleagues he saw Mr 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," Mr Kelly said, according to Mr Woodward.

"I don't even know why any of us are here. This is the worst job I've ever had," Mr Kelly added.

The looming release of Mr Woodward's book, according to reports, has had the White House on edge for weeks.

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 Sept 11.