US Secretary of State John Kerry has said tests showed that sarin nerve gas was used in a deadly 21 August chemical attack near Damascus.
Mr 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 US Congress - a decision that puts any strike on hold for at least nine days.
"This is squarely now in the hands of Congress," Mr Kerry told CNN, saying he had confidence "they will do what is right because they understand the stakes."
Mr Kerry declined to say whether Mr Obama would go ahead with military action if Congress rejects the president's request.
But echoing Mr 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.
With lawmakers due to be briefed by Mr Obama's national security team on the administration's rationale for military action, Mr 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," Mr 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 US intelligence agencies said killed more than 1,400 people, many of them children.
Pope calls for peace
Meanwhile in an address in Rome, Pope Francis made an impassioned plea for peace in Syria urging dialogue and talks rather than war.
Addressing thousands of people in St Peter's Square, he made a lengthy, detailed appeal for an end to conflict in Syria and across the Middle East.
God and history would be the judge of those who promoted violence or prevented peace, he said.
Pope Francis condemned the use of chemical weapons, blamed by Western powers on Syrian government forces which they deny, but added: "War, never again."
The US and France are considering military action against Damascus.
Pope Francis urged every effort to bring about peace based on "dialogue and negotiations".
"Violence never leads to peace, war leads to war, violence leads to violence," he said.