U.S. President Donald Trump wanted to assassinate Syrian President Bashar Assad after the April 2017 chemical attack on Idlib, journalist Bob Woodward writes in his upcoming book.

Trump called U.S. Defense Secretary James Mattis and said "Let’s fucking kill him! Let’s go in. Let’s kill the fucking lot of them," The Washington Post reported Tuesday in a piece with details from "Fear: Trump in the White House."

The chemical attack, which was carried out by Assad, killed dozens and prompted a U.S. missile strike against a Syrian air base which Washington said was used to launch the strike.

"Mattis told the president that he would get right on it. But after hanging up the phone, he told a senior aide: 'We’re not going to do any of that. We’re going to be much more measured.' The national security team developed options for the more conventional airstrike that Trump ultimately ordered," according to the report.

The alleged assassination plot is only one of many explosive revelations included in Woodward’s book on Trump’s first 18 months in office.

According to the journalist, Trump’s chief of staff privately called him an “idiot” and presidential aides plucked sensitive documents off Trump’s desk and thought he was often unaware of foreign policy basics.

Woodward quotes an exasperated Chief of Staff John Kelly doubting Trump’s mental faculties, declaring during one meeting, “We’re in Crazytown.” The book shows that both Secretary of Defense Jim Mattis and Kelly privately trashed their boss as incompetent, even viewing him as a threat to national security.

Woodward also quotes Trump saying to Rudy Giuliani: "Rudy, you're a baby," Trump told the man who is now his personal attorney. "I've never seen a worse defense of me in my life. They took your diaper off right there. You're like a little baby that needed to be changed. When are you going to be a man?"

And, Trump saying about Attorney General Jeff Sessions: "This guy is mentally retarded. He’s this dumb Southerner. … He couldn’t even be a one-person country lawyer down in Alabama."

Chief of Staff John Kelly on Trump: "He’s an idiot. It’s pointless to try to convince him of anything. 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."

The White House didn’t immediately respond to a request for comment.