WASHINGTON (AP) -- Secretary of State John Kerry on Monday outlined the clearest justification yet for U.S. military action in Syria, saying there was "undeniable" evidence of a large-scale chemical weapons attack, with intelligence strongly signaling that Bashar Assad's regime was responsible.
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.
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 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.
"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.
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.
"Anyone who can claim that an attack of this staggering scale can be contrived or fabricated needs to check their conscience and their own moral compass," he said.
British Prime Minister David Cameron, who like Kerry cut short his vacation because of the attack, spoke Monday with Russian President Vladimir Putin to outline the evidence of chemical weapons use by Assad's regime.
Cameron's office also said the British government would decide on Tuesday whether the timetable for the international response means it will be necessary to recall lawmakers to Parliament before their scheduled return next week. That decision could offer the clearest indication of how quickly the U.S. and allies plan to respond.
Late Monday, the State Department said it is postponing a meeting with Russian diplomats on Syria this week. The meeting at The Hague was about setting up an international conference to find a political resolution to the Syrian crisis. A senior State Department official said Monday the meeting between Undersecretary Wendy Sherman and U.S. Ambassador to Syria Robert Ford with their Russian counterparts was postponed because of the ongoing U.S. review about alleged use of chemical weapons in Syria.