U.S. Secretary of State John Kerry said tests had shown sarin nerve gas was fired on rebel-held areas near Damascus, and expressed confidence that lawmakers would do "what is right" in responding to last month's attack.
Washington says more than 1,400 people, many of them children, were killed in the attack.
Obama's decision on Saturday to seek congressional authorization for punitive military action against Syria is likely to delay any strike for at least nine days.
However, the United Nations said his announcement could be seen as part of an effort to forge a global consensus on responding to the use of chemical arms anywhere.
With Obama drawing back from the brink, President Bashar al-Assad's government reacted defiantly to the threat of Western retaliation for the August 21 chemical attack, which it says was staged by the rebels.
Assad said Syria was capable of confronting any external strike, but left the most withering comments to his official media and a junior minister.
"Obama announced yesterday, directly or through implication, the beginning of the historic American retreat," Syria's official al-Thawra newspaper said in a front-page editorial.
Syria's Deputy Foreign Minister Faisal Mekdad accused Obama of indecision. "It is clear there was a sense of hesitation and disappointment in what was said by President Barack Obama yesterday. And it is also clear there was a sense of confusion as well," he told reporters in Damascus.
Before Obama put on the brakes, the path had been cleared for a U.S. assault. Navy ships were in place and awaiting orders to launch missiles, and U.N. inspectors had left Syria after gathering evidence on the use of chemical weapons.
Kerry urged skeptical U.S. lawmakers to back a strike on Assad's forces. "This is squarely now in the hands of Congress," he told CNN, saying he had confidence "they will do what is right because they understand the stakes."
Last month's attack was the deadliest incident of the Syrian civil war and the world's worst use of chemical arms since Iraq's Saddam Hussein gassed thousands of Kurds in 1988.
However, opinion polls show strong opposition to a punitive strike among Americans weary of wars in Iraq and Afghanistan.
U.S. lawmakers for the most part welcomed Obama's decision but have not cut short their summer recess, which ends September 9. Many Democrats and Republicans are uneasy about intervening in a distant civil war in which 100,000 people have been killed over the past 2-1/2 years.
Lawmakers were to be