Earlier, Kerry insisted that the situation differs from Iraq, saying the intelligence community "reviewed and re-reviewed" its information "more than mindful of the Iraq experience." And he added: "We will not repeat that moment."
He cited particular evidence that he insisted shows al-Assad's regime was responsible.
"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," Kerry 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."
Additionally, the intelligence shows the day, time and location of the rockets that were launched and where and when they landed.
"We know rockets came only from regime-controlled areas and went only to opposition-controlled or contested neighborhoods," he said.
Quoting from the U.S. assessment, Kerry said the attack killed 1,429 people, including more than 400 children.
On Friday, the Syrian government called the U.S. intelligence information "old tales" based on "fabrications and lies."
'We are not alone'
Citing support from the Arab League, Turkey and France, Kerry said, "We are not alone in our will to do something" in response to the attack. He brushed off the British Parliament vote against joining a military invention, saying that the United States "makes our own decisions on our own timelines, based on our values and our interests" in deciding the proper course of action.
Meanwhile, the U.N. mission investigating the suspected chemical weapons attack in Syria has completed its collection of samples, said Martin Nesirky, spokesman for U.N. Secretary-General Ban Ki-moon.
Nesirky told reporters that inspectors visited a government military hospital in Damascus and the last of them will leave Syria on Saturday.
Ban will get a briefing Saturday from the inspectors, but a Western diplomat told CNN that the secretary-general would likely wait to meet again with the Security Council until a final report with laboratory analysis is completed, which could take a week.
Even as the U.N. inspection was winding down, opposition activists said Friday there is evidence of another deadly assault in Syria involving an incendiary agent. Seven people died and dozens were injured Monday in the attack on a school in northern Syria.
So far, opposition by Russia to any military response has scuttled U.N. action, and Kerry expressed little hope for a breakthrough.
"Because of the guaranteed Russian obstructionism of any action through the U.N. Security Council, the U.N. cannot galvanize the world to act as it should," he said.
Later Friday, Obama spoke by phone with French President Francois Hollande and British Prime Minister David Cameron. White House statements said in both calls, the leaders agreed the Syrian violation of chemical weapons bans cannot be tolerated, but only the statement on the call with Hollande said they agreed the Syrian regime must be held accountable.
While the British vote was a blow to Obama's hopes of getting strong support from key NATO allies and some Arab League states, regional NATO ally Turkey on Friday backed the U.S. contention that al-Assad's regime was responsible for the chemical attack.
"The information at hand indicates that the opposition does not have these types of sophisticated weapons," said Foreign Minister Ahmet Davutoglu. "From our perspective, there is no doubt that the regime is responsible."
Australia also weighed in, with Prime Minister Kevin Rudd saying the evidence against al-Assad was overwhelming and, "therefore, the focus now legitimately lies on the most appropriate from of international response."
Rudd said there has been "no request from the United States or any other country for a direct or indirect Australian military participation" in a possible strike against Syria.
Alone or together?
The White House has made clear that the United States will respond in some form to the use of chemical weapons. Previously, it ruled out U.S. troops on the ground or imposing a no-fly zone.
Sources have indicated a campaign of limited strikes by cruise missiles fired from U.S. naval ships in the region, targeting military command centers but not chemical weapons stockpiles, is the likely option.
The British Parliament vote and demands by other key European allies, including France and Germany, to put off a decision until after the U.N. inspectors report on what happened in Syria have slowed the response time.
Hollande told Le Monde newspaper that intervention should be limited and not be directed toward al-Assad's overthrow, a position also expressed by Obama.