"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.