US Secretary of State John Kerry delivered a passionate statement on the situation in Syria Friday, accusing the Bashar Assad regime of committing a crime against humanity in last week’s chemical weapons attack on the suburbs of Damascus, and saying it had to be held accountable.

He cited evidence from “thousands of sources” that went into a US intelligence report released as he was speaking. He said America was certain that the Assad regime had carried out the attack, which he said killed 1,429 Syrians.

“The United States government now knows that at least 1,429 Syrians were killed in this attack, including at least 426 children. Even the first-responders, the doctors, nurses and medics who tried to save them, they became victims themselves. We saw them gasping for air, terrified that their own lives were in danger. This is the indiscriminate, inconceivable horror of chemical weapons. This is what Assad did to his own people,” said Kerry.

Kerry said, “history would judge us all extraordinarily harshly if we turned a blind eye to a dictator’s wanton use of weapons of mass destruction against all warnings, against all common understanding of decency, these things we do know.”

The top American diplomat mentioned Iran specifically in warning that other nations and terror organizations were watching to see whether or not the US and its allies would respond.

“It is about whether Iran, which itself has been a victim of chemical weapons attacks, will now feel emboldened in the absence of action to obtain nuclear weapons. It is about Hezbollah and North Korea and every other terrorist group or dictator that might ever again contemplate the use of weapons of mass destruction,” he said.

“Will they remember that the Assad regime was stopped from those weapons’ current or future use? Or will they remember that the world stood aside and created impunity?”

Any US action would not involve boots on the ground or any open-ended engagement in Syria, according to Kerry, who said the US has no interest in assuming responsibility for the ongoing civil war.

The secretary of state said that the US was in the process of making decisions to ensure that “most heinous weapons must never again be used against the world’s most vulnerable people.”

Kerry was detailed and specific on what the US knew of the Assad regime’s chemical weapons attack last Wednesday: “We know that for three days before the attack, the Syrian regime’s chemical weapons personnel were on the ground in the area, making preparations,” he said.

“And we know that the Syrian regime elements were told to prepare for the attack by putting on gas masks and taking precautions associated with chemical weapons. We know that these were specific instructions.

“We know where the rockets were launched from, and at what time. We know where they landed, and when. We know rockets came only from regime-controlled areas and went only to opposition-controlled or contested neighborhoods.

“And we know, as does the world, that just 90 minutes later all hell broke loose in the social media. With our own eyes we have seen the thousands of reports from 11 separate sites in the Damascus suburbs. All of them show and report victims with breathing difficulties, people twitching with spasms, coughing, rapid heartbeats, foaming at the mouth, unconsciousness, and death. And we know it was ordinary Syrian citizens who reported all of these horrors.”

How the US responded, said Kerry, “has great consequences…

“It matters to our security and the security of our allies. It matters to Israel. It matters to our close friends Jordan, Turkey and Lebanon, all of whom live just a stiff breeze away from Damascus. It matters to all of them where the Syrian chemical weapons are — and if unchecked they can cause even greater death and destruction to those friends.

“And it matters deeply to the credibility and the future interests of the United States of America and our allies. It matters because a lot of other countries, whose policy has challenged these international norms, are watching. They are watching. They want to see whether the United States and our friends mean what we say.

“It is directly related to our credibility and whether countries still believe the United States when it says something. They are watching to see if Syria can get away with it, because then maybe they too can put the world at greater risk.

“And make no mistake, in an increasingly complicated world of sectarian and religious extremist violence, what we choose to do or not do matters in real ways to our own security. Some site the risk of doing things. But we need to ask, “What is the risk of doing nothing?”

“It matters because if we choose to live in the world where a thug and a murderer like Bashar al-Assad can gas thousands of his own people with impunity, even after the United States and our allies said no, and then the world does nothing about it, there will be no end to the test of our resolve and the dangers that will flow from those others who believe that they can do as they will.”

See the full text of Kerry’s statement here.