The Organization for the Prohibition of Chemical Weapons said Syria used the chemical weapons sarin and chlorine in a series of attacks in 2017.
At least six Kurds, including children, reportedly suffered severe burns.
The democratic presidential hopeful said the Syrian ruler should be held accountable "if the evidence is there" on chemical weapons.
Government officials claim insurgents were behind the gas attack, but rebel groups have called the allegations "purely a lie."
The attack in February left 11 people in need of emergency treatment.
The cynicism Americans feel about humanitarian intervention makes sense. But we must stand in solidarity with Syrians.
The U.S. also said Russians or Syrians may have tampered with evidence on the ground. Moscow denied the charge.
U.S. President Donald Trump launched the strikes in response to a deadly chemical attack near Damascus.
“Could not have had a better result,” said President Donald Trump. “Mission Accomplished!”
A military response to Assad's latest use of chemical weapons is unlikely to prevent more killing or diminish the influence of Russia and Iran.