Kerry, speaking to reporters at the State Department, said last week's attack was a "moral obscenity" that "should shock the conscience" of the world.
"The indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons is a moral obscenity. By any standard, it is inexcusable and -- despite the excuses and equivocations that some have manufactured -- it is undeniable," said Kerry, the highest-ranking U.S. official to confirm the attack in the Damascus suburbs that activists say killed hundreds of people.
"This international norm cannot be violated without consequences," he said.
Officials said President Barack Obama has not decided how to respond to the use of deadly gases, a move the White House said last year would cross a "red line." But the U.S., along with allies in Europe, appeared to be laying the groundwork for the most aggressive response since Syria's civil war began more than two years ago.
The U.S. and its allies appear to be considering a response that would punish Assad for deploying deadly gases, not sweeping actions aimed at ousting Assad or strengthening rebel forces. The focus of the internal debate underscores the scant international appetite for a large-scale deployment of forces in Syria and the limited number of other options that could significantly change the trajectory of the conflict.
The international community was moving ahead even as a United Nations team already on the ground in Syria collected evidence from last week's attack. The U.S. said Syria's delay in giving the inspectors access rendered their investigation meaningless and officials said the administration had its own intelligence confirming chemical weapons use.
"What is before us today is real and it is compelling," Kerry said. "Our understanding of what has already happened in Syria is grounded in facts."
The U.S. assessment is based in part on the number of reported victims, the symptoms of those injured or killed and witness accounts. Kerry said the administration also had additional intelligence and would make its findings public soon.
Syrian President Bashar Assad has denied launching a chemical attack.
Associated Press writer Deb Riechmann contributed to this report.
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