U.S. Secretary of State John Kerry said on Sunday tests showed that sarin nerve gas was used in a deadly Aug. 21 chemical attack near Damascus as he sought to build the case to convince skeptical lawmakers to authorize a military strike against the Syrian government.
Kerry made the disclosure in a series of television interviews a day after President Barack Obama delayed imminent military action in Syria to seek approval first from the U.S. Congress - a decision that puts any strike on hold for at least nine days.
"This is squarely now in the hands of Congress," Kerry told CNN, saying he had confidence "they will do what is right because they understand the stakes."
Kerry declined to say whether Obama would go ahead with military action if Congress rejects the president's request. But echoing Obama's comments in the White House Rose Garden on Saturday, he insisted that the president had the right to act on his own if he chooses that course.
Obama is taking a gamble by putting the brakes on an military assault that he has made clear is essential to maintain U.S. credibility regarding enforcement of a "red line" he set against the use of chemical weapons by Syrian President Bashar al-Assad's forces.
But the move reflects a desire to make sure Congress shares any responsibility for intervening in Syrian's civil war at a time when Americans are weary of wars in Iraq and Afghanistan.
With lawmakers due to be briefed by Obama's national security team on the administration's rationale for military action, Kerry used a round of television appearances to provide further evidence backing its accusations against the Syrian government.
"I can share with you today that blood and hair samples that have come to us through an appropriate chain of custody, from east Damascus, from first responders, it has tested positive for signatures of sarin," Kerry told CNN's "State of the Union."
It was the first time the administration has pinpointed what kind of chemical was used in the attack on a rebel-held area, which U.S. intelligence agencies said killed more than 1,400 people, many of them children.
"So this case is building and this case will build," Kerry told NBC's "Face the Nation."
Backing from Congress is by no means assured, with many Democrats and Republicans uneasy about intervening in a distant civil war in which 100,000 people have been killed over the past 2 1/2 years.
Lawmakers for the most part welcomed Obama's decision but looked in no hurry to come back to Washington early from their summer recess, which lasts until Sept. 9.
Seeking to lay the ground work for what is expected to be a heated congressional debate, Kerry repeatedly invoked Israel's security as a crucial reason to authorize a military response in Syria. Lawmakers of both political parties recognize how important it is to be seen as defenders of Israel whose security is often a key foreign policy issue in domestic election campaigns.
Mike Rogers, Republican chairman of House intelligence committee, told CNN: "I think there are some real challenges. I think that at the end of the day Congress will rise to the occasion. This is a national security issue."
Republican Senator Rand Paul of Kentucky, speaking on NBC, took a more skeptical view.
While saying that he was "proud" of Obama for coming to Congress for authority, Paul said: "It's at least 50-50 whether the House will vote down the involvement in the Syrian war."
"I think the Senate will rubber stamp what he wants," he said. "The House will be a much closer vote."