Press secretary Sean Spicer said an "extremely alarmed" Trump was briefed extensively by security aides on the suspected attack, which killed dozens in the rebel town of Khan Sheikhun.
"Today's chemical attack in Syria against innocent people, including women and children, is reprehensible," Spicer said in a prepared statement.
The administration, Spicer said, was "confident" that Assad was to blame but refused to speculate on how the US would respond.
"I'm not ready to talk about our next step, but we will get there soon," he said.
The Syrian army and Russia have categorically denied involvement.
Since coming to office in January, Trump had softened US criticism of the regime in Damascus -- which owes its survival to Russian and Iranian government backing.
Earlier this week Spicer indicated that the White House no longer saw Assad's departure as necessary for peace.
But amid diplomatic pressure from European allies in Britain and France, the White House signaled a tougher line.
Spicer said it was a "political reality" that Assad is in power and there was no "fundamental option of regime change," but he suggested it was in the "best interest" of Syrians that Assad not be in power.
"The idea that someone would use chemical weapons on their own people, including women and children, is not something that any civilized nation should sit back and accept or tolerate," he said.
"I think it's in the best interest of the Syrian people to not have anybody who would do the kind of heinous acts," said Spicer.
"Any leader who treats their people with this kind of activity, death and destruction. Yeah. I don't think anyone would wish this upon anybody."
The United Nations Security Council is scheduled to meet Wednesday to discuss the attack on Khan Sheikhun.
In February, Russia vetoed sanctions against Syrian officials blamed for previous chemical attacks, with little White House pushback.
But the Trump administration and the Kremlin could now be on a collision course.
President Vladimir Putin has long seen the Assad regime as a vehicle to strengthen Russia's foothold in the Middle East and expose the US as unable or unwilling to defend its allies and interests.
'Weakness and irresolution'
Republican Senator John McCain said the latest chemical attack showed the White House could not allow Assad to remain in place.
"In case it was not already painfully obvious: the notion that the Syrian people would be able to decide the fate of Assad or the future of their country under these conditions is an absurd fiction," he said.
"The recent statements by US officials suggesting otherwise only serve to legitimize the actions of this war criminal in Damascus."
But the White House rejected the notion that its initial stance on Syria may have emboldened Assad's regime to carry out Tuesday's attack.
Spicer instead deflected blame onto Barack Obama's administration.
Obama famously drew a red line over Assad's chemical weapons use, only to backtrack and negotiate a deal with Russia to take Syria's chemical stockpile off the battlefield.
Despite the agreement, some crude chemical weapons remain in use.
"These heinous actions by the Bashar al-Assad regime are a consequence of the past administration's weakness and irresolution," Spicer said.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
Get the latest election news, live updates and election schedule for Lok Sabha Elections 2019 on ndtv.com/elections. Like us on Facebook or follow us on Twitter and Instagram for updates from each of the 543 parliamentary seats for the 2019 Indian general elections.