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

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

Recalling that he had ordered cruise missile strikes on Assad for using chemical weapons, Trump said: "I am not somebody that will stand by and let him get away with what he tried to do."

Trump also claimed Syria would be different today had his predecessor Barack Obama taken action against Assad after drawing a "red line in the sand" over his regime's use of chemical weapons.

"Had president Obama gone across that line and done what he should have done, I don't believe you'd have Russia, and I don't believe you'd have Iran, anywhere near the extent and maybe not at all, in Syria today," he said.