Noting the U.N. team's mandate was only to determine the use of chemical weapons, and not identify who used them, Obama repeated past statements that U.S. intelligence has confirmed chemical weapons use beyond any reasonable doubt and has further confirmed that al-Assad's regime "was the source."
"I do think that we have to act, because if we don't, we are effectively saying that even though we may condemn it and issue resolutions and so forth and so on, somebody who is not shamed by resolutions can continue to act with impunity," Obama said.
International norms then "begin to erode," he added, and "other despots and authoritarian regimes can start looking and say, 'that's something we can get away with.'"
He described the intended U.S. response as "limited in time and in scope, targeted at the specific task of degrading (al-Assad's) capabilities, and deterring the use of those weapons, again."
More than 100,000 killed in Syrian conflict
The United Nations has said more than 100,000 people -- including many civilians -- have been killed since the popular uprising spiraled into a civil war in 2011.
In Washington, the resolution passed by the Senate Foreign Relations Committee set a 60-day deadline for use of force in Syria, with an option for an additional 30 days.
An amendment accepted by the panel from Republican Sen. John McCain of Arizona and Democratic Sen. Christopher Coons of Delaware added language to say the military response was intended to reverse Assad's battlefield momentum, a stronger objective than degrading the Syrian regime's chemical weapons capabilities as Hagel told the committee on Tuesday.
If Assad "remains in an advantageous position, he will never leave Syria," said McCain, who has been pushing for a more robust U.S. response. "He has to know that he is losing and that way you get a negotiated settlement for his departure."
The resolution also makes clear there would be no U.S. boots on the ground as part of a response in Syria.
A White House statement welcomed the panel's vote, saying "the military action authorized in the resolution would uphold America's national security interests by degrading Assad's chemical weapons capability and deterring the future use of these weapons, even as we pursue a broader strategy of strengthening the opposition to hasten a political transition in Syria."
After the vote, senators on the panel made statements that explained their thinking, with some calling for more efforts to build an international coalition before any attack takes place.
"Vietnam started with U.S. advisers and a limited Naval presence. It led to an all-out war," noted Democratic Sen. Tom Udall of New Mexico, who opposed the resolution.
Kerry, Hagel and Dempsey appeared before the Senate panel on Tuesday to press for approval of authorization.
Tough questioning by House panel
The same trio then faced questions on Wednesday before the House Foreign Affairs Committee, with Royce saying the administration's Syria policy had been adrift for two years.
At the same time, Royce acknowledged there were "no easy answers" on Syria, and attempting to deter chemical weapons use was worth considering despite public skepticism for U.S. military involvement.
Kerry said Obama sought authorization for a response to the use of banned weapons, not a full military intervention.
"We are not asking America to go to war," he said. "We all agree, there will be no American boots on the ground."
In response to a question, Hagel put the cost of the limited response under consideration at "tens of millions" of dollars.
Most of the focus of administration lobbying has been on the House, where opposition by liberal Democrats and libertarian conservatives, as well as the bitterly partisan political environment of the Republican-led chamber, make passage of Obama's authorization proposal uncertain.
House Speaker John Boehner and Cantor, the No. 2 Republican, both have endorsed a U.S. military response, but Wednesday's hearing showed widespread concerns and outside opposition from across the political spectrum.
Polls also show that a majority of Americans oppose a U.S. military strike on Syria.
In the Senate, a Democratic source familiar with Majority Leader Harry Reid's thinking told CNN that Reid is confident any authorization measure will pass his chamber. The source said it is likely 60 votes will be needed to overcome a filibuster, and Reid thinks the votes are there.
Before that, however, lawmakers will hear from the Russian government, which is moving ahead with its efforts to lobby Congress in an attempt to undercut Obama on Syria. Moscow has sent an official request to congressional leaders to meet with them.