At the same hearing, Kerry said any diplomacy "cannot be a process of delay. This cannot be a process of avoidance."
He later added that any agreement must include binding consequences if Syria fails to comply, and lawmakers moved to rewrite pending legislation along the same lines.
The president readied his speech as a small crowd of anti-war protesters, some waving signs, gathered outside the gates of the White House.
U.S. officials say more than 1,400 died in the Aug. 21 episode, including at least 400 children, and other victims suffered uncontrollable twitching, foaming at the mouth and other symptoms typical of exposure to chemical weapons banned by international treaty. Other casualty estimates are lower, and Assad has said the attack was launched by rebels who have been fighting to drive him from power in a civil war that has so far claimed the lives of more than 100,000 civilians.
Assad's patron, Russia, has blocked U.S. attempts to rally the Security Council behind a military strike. But Monday, after a remark by Kerry, it spoke favorably about requiring Syria to surrender control of its chemical weapons, and the Syrian foreign minister did likewise.
The foreign minister, Walid al-Moallem, said Tuesday that his government was ready to turn over its chemical weapons stockpile in line with Russia's proposal in order "to thwart U.S. aggression." He also said Syria was prepared to sign an international chemical convention it has long rejected -- a step it can take on its own at any time without U.S. or U.N. supervision.
Syria has long refused to provide an accounting of the size of its stockpile, rarely referring in public to its existence. According to an unclassified estimate by the French government, it includes more than 1,000 tons of "chemical agents and precursor chemicals," including sulfur mustard, VX and sarin gas.
Obama has said frequently he has the authority as commander in chief to order a military strike against Assad regardless of any vote in Congress.
The response in Congress to support such a strike has been lukewarm at best -- as underscored during the day when liberal Sen. Ed Markey, D-Mass., and conservative Rep. Mick Mulvaney, R-S.C., both announced their opposition.
Markey, who was elected to the seat that Kerry vacated when he joined the Cabinet, said the legislation under consideration was too broad, "the effects of a strike are too unpredictable, and ... I believe we must give diplomatic measures that could avoid military action a chance to work."
And Rep. James Langevin, a Rhode Island Democrat who sits on committees dealing both with military and intelligence matters, said he feared that "Iran and Russia could cause serious damage" to the United States if they retaliated with a cyberattack.
But Maryland Rep. Steny Hoyer, the second-ranking Democrat in the House, said, "It would be inimical to our country's standing if we do not show a willingness to act in the face of the use of chemical weapons and to act in a limited way to address that use alone."
Earlier, Senate Republican leader Mitch McConnell became the first congressional leader to come out against legislation giving the president authority for limited strikes. "There are just too many unanswered questions about our long-term strategy in Syria," he said.
By contrast, Speaker John Boehner of Ohio and Majority Leader Eric Cantor of Virginia, the top two Republicans in the House, have endorsed Obama's request.
Given the uncertainty of diplomatic maneuvering, no vote is expected for several days, if then.
"If something can be done diplomatically, I'm totally satisfied with that. I'm not a blood-and-thunder guy. I'm not for 'shock and awe,'" said Senate Majority Leader Harry Reid in a reference to the massive display of firepower that opened the war in Iraq nearly a decade ago.
Still, there was ample skepticism in Congress about the United Nations, Russia's true intentions and Syria's willingness to be bound by international agreements.
"There is an overwhelming view it would be preferable if international law and the family of nations could strip Syria of the chemical weapons," said Sen. Chuck Schumer, D-N.Y. "And there's a large view we should let that process play out for a little while."
Said Boehner: "Clearly, diplomacy is always a better outcome than military action. But I will say that I'm somewhat skeptical of those that are involved in the diplomatic discussions today."