"This case is building and this case will build," Kerry said on NBC's "Meet The Press."
He said emergency officials on the ground in Syria have determined from blood and hair samples that sarin was unleashed on a suburb of Damascus on Aug. 21, killing almost 1,500 people, including hundreds of children.
American officials learned of the lab results over the weekend, added Kerry, who pointed to the revelations as further impetus for Congress to quickly authorize a military intervention against Assad.
"We know the regime ordered this attack," he said. "We know the damage that was done afterward."
The White House had given every indication it was on the brink of acting on its own against Syria, dispatching Kerry to make a robust case for immediate intervention in a statement from the State Department on Friday. President Barack Obama had insisted previously that the use of chemical weapons by Assad would be considered a "red line" that the U.S. would not allow him to cross.
But just hours after Kerry made his statement, Obama did an abrupt about-face following a 45-minute walk on the White House grounds with his chief of staff, Dennis McDonough. On Saturday, he announced he would seek authorization to act in Syria from a notoriously fractured and obstructionist Congress.
Prior to his change of heart, Obama's national security advisers had reportedly not known that seeking congressional approval was even on the table for the president. NBC News reported the advisers were content that the only congressional consultation required was providing intelligence briefings to lawmakers, in part because no leaders in Congress were demanding a vote on Syria.
Kerry denied being caught off guard by the president.
"We argued — not argued, discussed — the options," he said. "The president then made the decision that he thought we would be stronger and the United States would act with greater moral authority and greater strength if we acted in a united way."
Kerry added: "The Congress is going to do what's right here."
There's no consensus, however, on how lawmakers — some of whom fumed publicly last week at the prospect of Obama launching military action without congressional approval — might vote in terms of a Syrian intervention.
One Republican congressman, Pete King of New York, even released a statement harshly criticizing Obama for seeking congressional authorization.
"President Obama is abdicating his responsibility as commander-in-chief and undermining the authority of future presidents," King said. "The president does not need Congress to authorize a strike on Syria .... The president doesn’t need 535 members of Congress to enforce his own red line."
King's fellow Republican Rand Paul, meantime, an anti-interventionist, praised the president's decision to consult Congress. But he added the situation in Syria is too rife with complications and shifting alliances for U.S. involvement.
"I think the war may escalate out of control and then we have to ask ourselves who is on America's side over there," Paul said on "Meet The Press."
"If the rebels win, will they be America's ally?"
He also used one of Kerry's famous anti-war quotes against him.
"He's famous for saying: 'How can you ask a man to be the last one to die for a mistake?' I would ask John Kerry: 'How can you ask a man to be the first one to die for a mistake?'"
Kerry asked the question while testifying at a congressional hearing in 1971 following his mililtary service in Vietnam.
American lawmakers are slated to return to the U.S. capital on Sept. 9 following their summer recess.
The White House opted against calling them back early due to Jewish holidays this week. That decision was backed by the Pentagon, which has told the administration that a delayed military response in Syria wouldn't hurt the U.S. in the Middle East.
The delay also gives Obama more than a week to exert public pressure on both Congress and the international community.
"Here's my question for every member of Congress and every member of the global community," the president said Saturday. "What message will we send if a dictator can gas hundreds of children to death in plain sight and pay no price?"