That evidence "includes but is not limited to" satellite images of activity at Syrian military installations identified as including chemical weapons depots, a senior administration official familiar with the intelligence told CNN.
And Kerry said he had argued to Syrian Foreign Minister Walid Moallem last week that if the government had nothing to hide, "then their response should be immediate: immediate transparency, immediate access, not shelling."
"Failure to permit that, I told him, would tell its own story," he said.
Al-Assad: It wasn't us
The Pentagon has sent four warships armed with cruise missiles to the region, and Obama will be presented with final options regarding actions against Syria in the next few days, a senior administration official said Monday.
But as U.S. muscle plows the waters of the eastern Mediterranean Sea, al-Assad on Monday repeated his government's denial that his army had anything to do with the use of poison gas.
"The area of the claimed attack is in contiguity with the Syrian army positions, so how is it possible that any country would use chemical weapons in an area where its own forces are located?" he asked in an interview with Russian newspaper Izvestia.
Al-Assad accused the United States, Britain and France of exploiting the incident by trying to verify rebel allegations instead of verifying facts.
The use of a large amount of chemical weapons would cross a "red line" and threaten U.S. interests in the region, Obama announced last year. Tabler, who has had extensive contacts with al-Assad, said the Syrian leader may have watched as Washington reacted cautiously to previous reports of chemical warfare and gambled incorrectly that last week's attack would draw a similar response.
"I think he thought that he could push the envelope again and that he could actually show his own people that no one is going to come to their rescue," Tabler said.
Opposition members say rockets with chemical payloads were among the ordnance government troops unleashed at the rebel stronghold of Ghouta early Wednesday. More than 1,300 people died, most of them by gas, according to opposition spokesman Khaled al-Saleh.
The opposition backed up the allegations with gruesome video of rows of dead bodies, including women and children. They had no visible wounds, and some appeared to be bloated. The aid agency Medecins sans Frontieres said three hospitals it supports in Syria's Damascus governorate reported having received about 3,600 patients displaying neurotoxic symptoms on Wednesday morning.
But according to Syrian state-run television's depiction of events, government forces came into contact with a gas attack on Saturday in Jobar, on the edge of Damascus. Several of the soldiers were "suffocating" from exposure to gases as they entered the city, according to state TV.
"It is believed that the terrorists have used chemical weapons in the area," Syrian TV reported, citing an anonymous source. The government uses the term "terrorists" to describe rebel forces.
Broadcast video showed a room containing gas masks, gas canisters and other paraphernalia that could be used in a gas attack. The army said it uncovered the cache in a storage facility in the area.
CNN could not independently confirm the authenticity of video shown by the government or rebels.