For almost two years, Obama has avoided direct military involvement in Syria's civil war, only escalating aid to rebel fighters in June after suspected smaller-scale chemical weapons attacks by Syrian government forces.
However, last week's attack obliterated the "red line" Obama set just over a year ago against the use of Syria's chemical weapons stocks.
Veteran Republican Sens. John McCain of Arizona and Lindsey Graham of South Carolina challenged Obama's plan Friday, issuing a statement that said the purpose of any U.S. military action in Syria "should not be to help the president save face."
The goal, they said, "should be to shift the balance of power on the battlefield against Assad and his forces."
Kerry: "We will not repeat" Iraq
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.