James Mattis said Trump has 'understanding of a fifth or sixth grader,' new book claims

James Mattis did not mince words about Donald Trump after a contentious National Security Council meeting, according to a new book by Bob Woodward.

In Fear, the veteran reporter highlighted a tense moment the US Secretary of Defence shared with his associates after the president questioned US military strategy in the Korean peninsula.

The Washington Post, which received an early copy of Mr Woodward’s book, writes: "At a National Security Council meeting on Jan. 19, Trump disregarded the significance of the massive U.S. military presence on the Korean Peninsula, including a special intelligence operation that allows the United States to detect a North Korean missile launch in seven seconds vs. 15 minutes from Alaska, according to Woodward. Trump questioned why the government was spending resources in the region at all.”

"We’re doing this in order to prevent World War III," Mr Mattis explained to Mr Trump, according to the book.

When the president left the room, Mr Woodward recalled the defence secretary appearing concerned and exhausted. He then reportedly told close associates that Mr Trump was acting like — and having the understanding of — a "fifth- or sixth-grader."

Mr Mattis has denied he uttered comments, saying a statement: “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.”

The Post — where Mr Woodward serves as an associate editor — also released audio from a phone call between the journalist and Mr Trump, who initially claimed to have never been told about the book before it was too late to provide commentary for it.

At one point, the president puts Kellyanne Conway on the phone after Mr Woodward said he made a request to interview the president with the top aide — who claimed to have placed that request with another official.

Mr Trump eventually admit he was told about the book and Mr Woodward’s attempts to contact him for an interview after the journalist said that he had spoken with Senator Lindsey Graham about the matter.

The book details how Mr Trump’s top advisers appear to go great lengths to protect national interests, either by working to sway the president’s thinking or even swiping documents from his table before he gets the chance to sign them.

“Secretaries of defence don’t always get to choose the president they work for,” Mr Mattis told his friends, according to Mr Woodward.

White House Chief of Staff John Kelly also reportedly expressed negative views about the president, calling Mr Trump “an idiot,” according to Mr Woodward.

“It’s pointless to try to convince him of anything,” the chief of staff said, according to the book. ”He’s gone off the rails. We’re in Crazytown. I don’t even know why any of us are here. This is the worst job I’ve ever had.”

In a statement, the White House called the new book "nothing more than fabricated stories, many by former disgruntled employees, told to make the President look bad."

Mr Kelly also rebuked the claims of his statements in the book, writing in a statement, "The idea I ever called the President an idiot is not true."