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

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

Trump's comments came during a White House press conference with Lebanon's Prime Minister Saad Hariri, who made his first official visit to the Trump administration on Tuesday.

"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 claimed Syria today had his predecessor Barack Obama taken action against Assad after drawing a "red line in the sand."

"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.

Trump also accused Iran and Hizballah, which the United States considers a "foreign terrorist organisation," of fueling the humanitarian disaster in Syria, claiming the Lebanese militant group is a threat to the entire Middle East.

"Hizballah 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 said. "With the support of Iran, the organisation is also fueling humanitarian catastrophe in Syria."

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

But Trump demurred when asked to comment on possible additional sanctions against the group, instead stating he plans to make his "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."

Meanwhile Trump praised the Lebanese army for "fighting continually to guard Lebanon's border and prevent IS and other terrorists... from gaining a foothold inside their country.

"The United States military has been proud to help in that fight and will continue to do so," he said. "America's assistance can help ensure that the Lebanese army is the only defender Lebanon needs," he said.

Although Trump pledged continued US military aid to Lebanon following his Oval Office meeting with Hariri, the current draft State Department budget calls for cuts in US military assistance and analysts say Lebanon could be among the countries affected.