Kerry's tough language marked the clearest justification yet for U.S. military action in Syria, which, if President Barack Obama decides to approve, most likely would involve sea-launched cruise missile attacks on Syrian military targets.
Speaking to reporters at the State Department on Monday, Kerry was harshly critical of chemical warfare.
"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.
Obama has not decided how to respond to the use of deadly gases, officials said. The White House said last year that type of warfare would cross a "red line." 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 on Tuesday, 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 the Syrian leader 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 Obama administration 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. U.N. officials disagreed that it was too late.
"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. 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.
Assad has denied launching a chemical attack. The U.N. team came under sniper fire Monday as it traveled to the site of the Aug. 21 attack.
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.
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. The president 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.
The U.S. Navy last week moved a fourth destroyer into the eastern Mediterranean. Each ship can launch ballistic missiles.
Officials said it was likely the targets of any cruise-missile attacks would be tied to the regime's ability to launch chemical weapons attacks. Possible targets would include weapons arsenals, command and control centers, radar and communications facilities, and other military headquarters. Less likely was a strike on a chemical weapons site because of the risk of releasing toxic gases.
Military experts and U.S. officials said Monday that the precision strikes would probably come during the night and target key military sites.
The president has ruled out putting American troops on the ground in Syria and officials say they are not considering setting up a unilateral no-fly zone.
On Capitol Hill, bipartisan support for a military response appeared to be building, with some key lawmakers calling for targeted strikes. A spokesman for House Speaker John Boehner said the Ohio Republican had "preliminary communication" with White House officials about the situation in Syria and a potential American response.
It's unlikely that the U.S. would launch a strike against Syria while the United Nations team is still 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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