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 Shia armed group, 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 attacks 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".

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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 heightening 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 organisation 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."

Trump praised Hariri for his nation's effort in hosting Syrian refugees.

"I want to thank the prime minister and the Lebanese people for giving shelter to those victimised by ISIS [ISIL, Islamic State of Iraq and the Levant], the Assad regime and their supporters and sponsors, and pledge our continued support to Lebanon," Trump said at a press conference with Hariri.

Trump, who has sought to halt the acceptance of Syrian refugees into the United States and vowed cuts to US foreign aid programmes, stressed the importance of helping people close to home and said the US would continue to provide humanitarian support to efforts in Lebanon.

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"Our approach, supporting the humanitarian needs of displaced Syrian citizens as close to their home country as possible, is the best way to help most people," he said.

Hariri pointed to the challenges faced by his nation in hosting some 1.5 million displaced Syrians. Syrian refugees make up about 25 percent of Lebanon's population of about 6 million people.

Another item on the prime minister's agenda is continued US support for the Lebanese military.

Trump praised the Lebanese army for "fighting continually to guard Lebanon's border and prevent ISIS 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."

The current draft state department budget calls for cuts in US military assistance and analysts say Lebanon could be among the countries affected.