Trump denies claims he wanted to assassinate Syria's Assad

US President Donald Trump denied on Wednesday having discussed wanting to kill Syria's President Bashar al-Assad, a key claim in a new book by Washington Post journalist Bob Woodward.

"That was never even contemplated," Trump told reporters in the Oval Office as he met with the emir of Kuwait.

Trump's comments came as he warned Damascus that "the world is watching" Syrian troops massing on the edges of the rebel-held province of Idlib, raising fears of a humanitarian disaster.

"I just tell you that they will hopefully be very, very judicious and careful," Trump said of the Syrian regime.

"Because the world is watching. That cannot be a slaughter. If it's a slaughter, the world is going to be very, very angry. And the United States is going to be very angry, too."

In his new book "Fear: Trump in the White House," Woodward recounts that the president told Defense Secretary Jim Mattis that he wanted to have Assad killed after he carried out a chemical attack on civilians in April 2017.

"Let's f*cking kill him! Let's go in. Let's kill the f*cking lot of them," Woodward quotes Trump as saying.

He writes that Mattis told the president he would "get right on it" but then came back later with plans for a more limited air strike.

Trump and White House aides have assailed the book as a full of "made up" stories, and in a tweet on Wednesday morning the president wondered why Congress doesn't change the laws on libel.

"The book means nothing. It's a work of fiction," Trump told reporters in the Oval Office, noting that Mattis and White House chief of staff John Kelly had both issued statements rejecting the book's account.