Mr Trump believes dictator Bashar al-Assad's forces were behind the sarin gas attack in Idlib, which left at least 89 people dead.
Assad's regime and his allies, including Vladimir Putin, have repeatedly denied the claims, instead blaming terrorists or rebel groups.
The absolute mass of data we have is too massive to fabricate
White House spokesman
The official added: "The cover-up is the disinformation that has happened since the day of the attack.
"The absolute mass of data we have is just too massive for any intelligence organisation to fabricate in that period of time."
His comments are likely to further heighten tensions between Russia and the United States.
Donald Trump believes Assad was behind last week's sickening attack
Horrific images depict sickening chemical gas attack in Syria
Tue, April 4, 2017
At least 58 people, including nine children, were killed in an air raid that released "toxic gas" on the rebel-held Syrian town of Khan Sheikhoun
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A child receiving treatment at a field hospital after an alleged chemical attack in Idlib, northen Syria
Assad and his allies have blamed terrorists or rebel groups
In the wake of the sickening incident, Mr Trump showed the US' military might by launching a missile attack against Mr Assad's forces.
The President's son Eric yesterday claimed the intervention proved Mr Trump will not be "intimidated" by Mr Putin.
He said: "He is not a guy who gets intimidated. I can tell you he is tough and he won't be pushed around.
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"The cards will shake out the way they do but he's tough.
"[The chemical attack] was horrible. These guys are savages and I'm glad he responded the way he responded.
"My father will act in times like that. I'm proud he took that action and believe me, he thinks things through."