Trump says won't let Assad get away with 'horrible' crimes

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 also called the Lebanese Shiite militia Hezbollah a threat to the entire Middle East.

"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.

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 said he believed there might not be any Russian or Iranian involvement in Syria today had former president Barack Obama taken action against Assad.

Trump accused Hezbollah and Iran of fueling the humanitarian disaster in Syria.

"Hezbollah is a menace to the Lebanese state, the Lebanese people and the entire region," he said.

"The group continues to increase its military arsenal which threatens to start yet another conflict with Israel," Trump added. "With the support of Iran, the organization is also fueling humanitarian catastrophe in Syria."

Hezbollah's "true interests are those of itself and its sponsor, Iran," he said.

Trump demurred when asked to comment on sanctions against Hezbollah.

"I'll be making my position very clear over the next 24 hours," he said. "I have meetings with some of my very expert military representatives and others, so I'll be making that decision very shortly."