President Donald Trump reportedly told Defense Secretary James Mattis he wanted to assassinate Syrian President Bashar al-Assad last year.

It came after a chemical weapons attack against Syrian civilians last year, according to an excerpt from Bob Woodward's new book.

Mattis reportedly told Trump he'd get "right on it," only to tell an aide their actual response would be "much more measured."

Woodward's new book details how Trump's top advisers often try to work around his impetuous nature.

President Donald Trump reportedly told Defense Secretary James Mattis he wanted to assassinate Syrian President Bashar al-Assad after a chemical weapons attack against Syrian civilians last year, according to an excerpt from author Bob Woodward's new book.

"Let’s f---ing kill him! Let’s go in. Let’s kill the f---ing lot of them," Trump said to Mattis on the phone after the chemical weapons attack in Douma, Syria, according to details of the book, "Fear," published by The Washington Post.

Mattis reportedly told Trump he'd get "right on it" in an apparent attempt to pacify the president. He simultaneously told a senior aide they would not be going down that road.

"We’re not going to do any of that. We’re going to be much more measured," Mattis told the aide at the time, Woodward wrote.

The Trump administration eventually responded with limited missile strikes against Assad, targeting chemical weapons sites in Syria. The strikes ultimately did little to damage Assad's military or chemical weapons capabilities.

The reported exchange between Trump and Mattis on the Syrian leader captures a broader theme in Woodward's forthcoming book: how Trump's top advisers often work around his impetuous nature.

According to Woodward, the defense secretary has marveled at Trump's ignorance on foreign affairs and told close associates the president has the intelligence of "a fifth- or sixth-grader."

Trump has been tweeting about the conflict in Syria in recent days as Assad's forces close in on the last major rebel stronghold in the Syrian city of Idlib.

"President Bashar al-Assad of Syria must not recklessly attack Idlib Province," Trump tweeted on Monday. "The Russians and Iranians would be making a grave humanitarian mistake to take part in this potential human tragedy. Hundreds of thousands of people could be killed. Don’t let that happen!"

Russian and Syrian fighter jets began conducting airstrikes near Idlib by Tuesday morning.

The White House responded later Tuesday to the book.

“This book is nothing more than fabricated stories, many by former disgruntled employees, told to make the President look bad," said Press Secretary Sarah Sanders in a statement.

White House chief of staff John Kelly decried "Fear" as “another pathetic attempt to smear people close to President Trump and distract from the administration’s many successes."