Kerry said findings are "clear and compelling," and that sources show the chemical weapons attack was inflicted by Assad regime. According to Kerry, the Syrian regime used chemical weapons multiple times during the last year.
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Kerry referred to a declassified intelligence report, according to which the Aug. 21 attack killed 1,429 Syrian civilians, including 426 children.
Kerry made clear that the United States will punish Assad for the "brutal and flagrant" chemical weapons attack, adding it was essential not to let Syria get away with the attack, partly as a sign to those who might consider using chemical weapons in the future, and said the United States was joined by allies including France, "our oldest ally," in its determination to act.
"It matters here if nothing is done," Kerry said. "It matters if the world speaks out in condemnation and then nothing happens."
"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," Kerry said.
Kerry also stressed that anything the United States might do would be carefully tailored and would not in any way resemble the U.S. invasions of Afghanistan and Iraq, nor its intervention to help topple former Libyan dictator Muammar Gaddafi.
"It will not involve any boots on the ground. It will not be open-ended. And it will not assume responsibility for a civil war that is already well underway," Kerry said of any action US President Barack Obama might pursue.
Kerry noted that he called the Syrian foreign minister and told him: "If, as you say, your nation has nothing to hide then let the United Nations in immediately and give the inspectors the unfettered access, so they have the opportunity to tell your story."
Kerry continued: "Instead, for four days, they shelled the neighborhood in order to destroy evidence, bombarding block after black at a rate four times higher than they had over the previous 10 days. And, when the UN inspectors finally gained access, that access - as we now know - was restricted and controlled."
However, Kerry said that the UN can't tell anything that the US hasn't shared this afternoon.
The declassified intelligence report released on Friday will be used by President Obama to make the case for retaliation against the Syrian government.
To read the intelligence report, click here
The intelligence gathered for the US report included an intercepted communication by a senior official intimately familiar with the attack as well as other human, signals and satellite intelligence, the four-page report said.
Earlier, President Barack Obama consulted on the Syrian issue with his national security team and with Kerry.
It should be noted that the inspectors' mandate was to determine whether chemical weapons were used, not to assign blame, and therefore the US is not waiting for their report before it issues its own on the Assad regime's responsibility.
On Thursday, the Britain released a similar report, according to which there's a high probability that the Syrian regime was behind the chemical attack. The report details the legal justifications for a military intervention in Syria by Britain, even if the UN Security Council objects.
However, on Thursday the British House of Commons voted against a possible military strike in Syria.
Prime Minister David Cameron on his part he regretted the failure of the British parliament to support military action in Syria but that he hoped President Barack Obama would understand the need to listen to the wishes of the people.
"I think the American public, the American people and President Obama will understand," Cameron said just hours after parliament voted against a government motion to authorise the principle of military action in Syria.
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