"I think what Assad did is terrible," Trump told reporters travelling with him on Air Force One en route to Florida.
"I think what happened in Syria is a disgrace to humanity and he's there, and I guess he's running things, so something should happen," Trump said.
The poison gas attack on Tuesday in the rebel-held Syrian town of Khan Sheikhoun killed at least 70 people, many of them children, and presented Trump with his biggest foreign policy crisis since he took office in January.
Washington has blamed the attack on the Syrian government, putting it at odds with Russia, which has air and ground forces in Syria.
Trump has until now focussed his Syria policy almost exclusively on defeating ISIS terrorists in northern Syria, where US Special Forces are working with Arab and Kurdish armed groups.
Only last week, US Ambassador to the United Nations Nikki Haley said the US diplomatic policy on Syria for now was no longer focussed on making Assad leave power.
The gas attack in Idlib province outraged the world. Trump said on Wednesday it had caused him to think again about Assad.
A US official said options being discussed by the Pentagon and White House after the gas attack could include grounding aircraft used by Assad's forces, the official added.
Such options would also include the use of cruise missiles, allowing the United States to strike targets without putting piloted aircraft in the skies above Syria.
The US official did not comment on how likely military action might be or suggest which, if any, options might be recommended by the Pentagon.
The official added that US Defence Secretary Jim Mattis and White House national security adviser HR McMaster had already been in talks on the matter.
Mattis will presumably discuss the options when he meets with Trump at his Mar-a-Lago retreat in Florida, the official said. Mattis is due in Florida later on Thursday as part of a scheduled trip.
US intelligence agencies suspect that Assad kept some of the chemical weapons or components that he agreed to surrender under a US-Russian deal that followed a chemical weapons attack on the outskirts of Damascus in 2013, a US intelligence official said on Thursday.
"We have never taken the Assad regime at its word that it declared its entreutersire chemical weapons stockpile," the official said.
Turkey would welcome US military action in Syria following the gas attack and would be ready to assist if needed, President Tayyip Erdogan was quoted as saying. Ankara has been a steadfast opponent of Assad and repeatedly called for his ouster.
French presidential candidate Emmanuel Macron said on Thursday a US-authorised international military force should intervene in Syria if it is proven that Assad's government used chemical weapons.
(Additional reporting by Roberta Rampton in Washington; Writing by Alistair Bell; Editing by Yara Bayoumy and Peter Cooney)
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