Syria's U.N. ambassador, Bashar al-Jaafari, told CNN's Christiane Amanpour that the allegations of poison gas use by his government "are false and unfounded." Al-Assad told the French newspaper Le Figaro on Monday that an attack on his country risks a regional war.
Administration officials and Obama himself have said the president has the authority to conduct military strikes even without congressional approval. But asked what the administration would do if Congress refused to authorize military action, Kerry said, "We're not contemplating that, because it's too dire."
The session was interrupted early on by a member of the anti-war group Code Pink, who shouted "The American people do not want this" as she was dragged out of the room by police.
Kerry first became famous decades ago as a former Navy officer testifying against the war in Vietnam in front of the same committee. He responded to the protest by saying that "Congress will represent the American people, and I think we all can respect those who have a different point of view."
Weapons inspectors from the United Nations are analyzing samples taken from the scene last week, but their results aren't expected to assign blame. The United States and its allies say the rebels have no capability to carry out a large-scale chemical weapons attack, however.
House leaders line up behind Obama
Earlier Tuesday, the leaders of both parties in the House of Representatives emerged from a White House meeting earlier Tuesday to support Obama's call for American strikes.
House Speaker John Boehner, R-Ohio, told reporters the use of poison gas was "a barbarous act" to which only the United States is capable of responding. House Minority Leader Nancy Pelosi, D-California, added that Washington must respond to actions "outside the circle of civilized human behavior."
But in a written statement later, Boehner said it is up to Obama "to make his case to the American people and their elected representatives" -- including securing support from individual members.
"All votes authorizing the use of military force are conscience votes for members, and passage will require direct, continuous engagement from the White House," the speaker said.
No vote is scheduled before Congress returns from its summer recess on Monday. Kerry defended the delay to seek congressional authorization, saying the move gives the United States time to make its case while adding pressure on al-Assad.
"This is working," Kerry said. "There are defections taking place. There's great uncertainty in Syria. We are building support, a greater understanding, and I would far rather be playing our hand than his at this point in time."
Most of the focus of administration lobbying has been on the House. 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.
U.N. chief calls for end to Security Council 'stalemate'
The United Nations has said more than 100,000 people -- including many civilians -- have been killed in Syria since a popular uprising spiraled into a civil war in 2011. Syrian opposition activists reported another 107 dead on Monday, mostly in Damascus and its suburbs.
New U.N. figures Tuesday point to the staggering impact the war has had on the nation.
The number of Syrians who have fled their war-ravaged country has risen above 2 million, the U.N. refugee agency reported, an increase of nearly 1.8 million people over the past 12 months.
But at the United Nations, Secretary-General Ban Ki-moon urged Security Council members to await test results on the samples collected by U.N. inspectors.
"We should avoid further militarization of the conflict and revitalize the search for a political settlement," Ban said. "I take note of the argument for action to prevent further uses of chemical weapons. At the same time, we must consider the impact of any punitive measure on efforts to prevent further bloodshed and facilitate a political resolution of the conflict. The turmoil in Syria and across the region serves nobody."
He urged member states to work through the Security Council, where Syrian allies Russia and China are expected to block any call for military action.