United States Secretary of State John Kerry addresses the media on the Syrian situation in Washington August 26, 2013. UN investigators took blood samples today from victims of an apparent poison gas attack that occurred on Wednesday in a rebel-held suburb of Damascus. [Photo/Agencies]

WASHINGTON — Secretary of State John Kerry on Monday outlined the clearest justification yet for US 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 US 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 US, 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 US 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 US 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 US 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 US 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.