Washington: US President Donald Trump says he did not discuss assassinating Syrian leader Bashar al-Assad, disputing an account in a forthcoming book by journalist Bob Woodward.

Trump, speaking to reporters during a visit with Kuwaiti ruler Sheikh Sabah al- Ahmad al-Jaber al-Sabah, said a possible assassination was "never even discussed".

"That was never even contemplated, nor would it be contemplated," Trump said, calling Woodward's book "fiction".

Trump wanted to have Assad assassinated last year but Defence Secretary James Mattis ignored the request, according to the book Fear: Trump in the White House.