Syrian opposition health minister Firas Jundi said more than 100 victims died as a result of a strike Monday on the outskirts of Khan Sheikhoun.
"Their deaths [were] an affront to humanity," Trump said during a joint press conference with Jordan's King Abdullah.
"These heinous actions by the Assad regime cannot be tolerated.”
The details surrounding the horrific incident are unclear, including which agent, if any, was deployed.
While the Organization for the Prohibition of Chemical Weapons begins an independent investigation, preliminary condemnations have squarely placed blame on Syrian President Bashar al-Assad's forces. But his principal backer, Russia, said a Syrian airstrike hit a rebel chemical weapons stockpile, resulting in collateral damage.
Trump has long been a skeptic of ousting Assad, and has publicly derided the opposition. But he said the attack Monday has changed his views -- at least with respect to Assad.
"That was a horrible, horrible thing," he said. "My attitude toward Syria and Assad has changed very much."
Trump adamantly declined to specify how his views have changed but his ambassador to the UN, Nikki Haley, signaled earlier Wednesday that the U.S. may take unilateral action if the Security Council fails to act.
"When the UN consistently fails in its duty to act collectively, there are times in the life of states when we are compelled to take our own action," Haley said.