Kerry, speaking to reporters at the State Department, said last week's attack "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 added.
In Damascus on Monday, U.N. experts collected samples and testimony from Syrian doctors and victims of the alleged chemical weapons attack, following a journey through government and rebel-held territory, where their convoy was hit by snipers. The U.N. said the team was safe.
Assad has denied launching a chemical attack. On Monday, his government vowed to defend itself against any international attack, warning that such an intervention would ignite turmoil across the region.
In Washington, 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.
Two administration officials said the U.S. was expected to make public a more formal determination of chemical weapons use today, with an announcement of Obama's response likely to follow quickly. The officials insisted on anonymity because they were not authorized to publicly discuss the internal deliberations.
The international community appeared to be considering action that would punish Assad for deploying deadly gases, not sweeping measures aimed at ousting him 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.
"We continue to believe that there's no military solution here that's good for the Syrian people, and that the best path forward is a political solution," State Department spokeswoman Marie Harf said. "This is about the violation of an international norm against the use of chemical weapons and how we should respond to that."
The U.S. said Syria's delay in giving the U.N. inspectors access rendered their investigation meaningless and officials said the administration had its own intelligence confirming chemical weapons use.
The U.S. assessment is based in part on the number of reported victims, the symptoms of those injured or killed and witness accounts. Administration officials said the U.S. had additional intelligence confirming chemical weapons use and planned to make it public in the coming days.
Officials stopped short of unequivocally stating that Assad's government was behind the attack. But they said there was "very little doubt" that it originated with the regime, noting that Syria's rebel forces do not appear to have access to the country's chemical weapons stockpile.
It's unclear whether Obama would seek authority from the U.N. or Congress before using force. The president has spoken frequently about his preference for taking military action only with international backing, but it is likely Russia and China would block U.S. efforts to authorize action through the U.N. Security Council.
Kerry on Monday made several veiled warnings to Russia, which has propped up Assad's regime, blocked action against Syria at the U.N., and disputed evidence of the government's chemical weapons use.
More than 100,000 people have died in clashes between forces loyal to Assad and rebels trying to oust him from power over the past two and a half years. While Obama has repeatedly called for Assad to leave power, he has resisted calls for a robust U.S. intervention, and has largely limited American assistance to humanitarian aid. Obama said last year that chemical weapons use would cross a "red line" and would likely change his calculus in deciding on a U.S. response.
Last week's attack in the Damascus suburbs is a challenge to Obama's credibility. He took little action after Assad used chemical weapons on a small scale earlier this year and risks signaling to countries like Iran that his administration does not follow through on its warnings.
Syrian activists say the Aug. 21 attack killed hundreds; the group Doctors Without Borders put the death toll at 355 people.
Assad told the Russian newspaper Izvestia that accusations his troops used chemicals were "politically motivated."
"This is nonsense," Assad was quoted as saying. "First they level the accusations, and only then they start collecting evidence."
Assad said attacking such an area with chemical weapons would not make sense for the government, because there was no clear front line between regime and rebel forces.
It's unlikely that the U.S. would launch a strike against Syria while the U.N.team is in the country. The administration may also try to time any strike around Obama's travel schedule -- he's due to hold meetings in Sweden and Russia next week -- in order to avoid having the commander in chief abroad when the U.S. launches military action.
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