US President Donald Trump accused Syria's leader Bashar al-Assad of committing "horrible" crimes against humanity Tuesday and vowed to prevent his regime from carrying out any more chemical attacks.

"I'm not a fan of Assad. I certainly think that what he's done to that country and to humanity is horrible," Trump said at a press conference with Lebanon's Prime Minister Saad Hariri.

"I am not somebody that will stand by and let him get away with what he tried to do, and he did it a number of times."