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(CNN) — U.S. intelligence proves “beyond any reasonable doubt” that Syrian President Bashar al-Assad’s government was behind the August poison gas attack outside Damascus, Secretary of State John Kerry told a Senate committee Tuesday.

Kerry, Defense Secretary Chuck Hagel and Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, appeared before the Senate Foreign Relations Committee as the Obama administration lobbied for Congress to authorize military action against Syria. From the outset, Kerry addressed the shadow of claims offered in the buildup to the 2003 invasion of Iraq, reassuring committee members that the intelligence linking Syrian government forces to the August 21 attack was solid.

“We are especially sensitive, Chuck and I, to never again asking any member of Congress to take a vote on faulty intelligence,” said Kerry, who like Hagel, voted as a senator to authorize the Iraq invasion. “And that is why our intelligence community has scrubbed and re-scrubbed the evidence. We have declassified unprecedented amounts of information, and to ask the American people and the rest of the world to judge that information.”

This time, Kerry said, “only the most willful desire to avoid reality” can deny that poison gas was used in the August 21 attack “and the Assad regime did it.”

Kerry said President Barack Obama isn’t asking the United States to go to war — just “to degrade and deter (Syrian President) Bashar al-Assad’s capability to use chemical weapons.”

The hearing was interrupted briefly 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 that “Congress will represent the American people, and I think we all can respect those who have a different point of view.”

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 that only the United States has “the capability and capacity” to respond to the Syrian gas attack.

“The use of chemical weapons is a barbarous act. It’s pretty clear to me that the United Nations is unable to take action; NATO, not likely to take action,” Boehner said.

House Minority Leader Nancy Pelosi added that Washington must respond to actions “outside the circle of civilized human behavior.”

“Humanity drew a line decades ago that I think if we ignore, we do so to the peril of many other people who could suffer,” said Pelosi, D-California.

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.

The United States, along with NATO and several other countries, blames al-Assad’s forces for a chemical weapons attack that’s believed to have killed more than 1,000 people — including, Obama said Tuesday, more than 400 children.

Syria denies the accusations and accuses rebel groups of using chemical weapons, while the rebels blame government troops. The United States and several of its allies say the rebels don’t have the capability to launch a large-scale chemical attack like the one on August 21.

At the United Nations, Secretary-General Ban Ki-moon urged Security Council members to await test results on samples collected by U.N. weapons inspectors who left Syria over the weekend.

“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.

“The Security Council has a duty to move beyond the current stalemate and show leadership,” Ban said. “This is a larger issue than the conflict in Syria. This is about our collective responsibility to humankind.”

Meanwhile, 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.” All Syrians will be victims “of any escalation of the situation,” al-Jaafari added.

“We don’t need wars. We need peaceful settlement of conflicts according to the charter of the United Nations,” he said.

Obama’s meeting with congressional leaders was part of his administration’s push to convince Congress to support action against Syria. Most of the focus 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.

Obama: The plan ‘fits into a broader strategy’

Obama said failing to punish Syria would send “a message that international norms around issues like nuclear proliferation don’t mean much,” he said. The U.S. military plan “gives us the ability to degrade Assad’s capabilities when it comes to chemical weapons.

“It also fits into a broader strategy that we have to make sure that we can bring about over time the kind of strengthening of the opposition and the diplomatic and economic and political pressure required so that ultimately we have a transition that can bring peace and stability, not only to Syria, but to the region.”

Obama said Saturday that the use of chemical weapons is “a challenge to the world” that threatens U.S. allies in the region — but he said he would seek the authorization of Congress before unleashing American force. No vote on military action is expected until after lawmakers reconvene from recess on September 9.

Five American warships, including four guided-missile destroyers, are now posted off Syria in the eastern Mediterranean Sea. Another five, including the aircraft carrier USS Nimitz, are in the Red Sea, but aren’t expected to take part in any strike, a U.S. official said Monday.

The threat of attack has, as expected, led to “a lot of dispersal” of Syria military assets in recent days, a U.S. official with direct access to the latest information told CNN. That means the U.S. military needs to update its target lists continuously, the official said.

But former CIA Director Michael Hayden said that also means it’s harder for al-Assad’s troops to operate on the battlefield.

“He’s dispersed his forces. He’s camouflaged his forces. He’s hidden his forces. That means he can’t use his forces,” Hayden, a retired Air Force general, told CNN’s “The Lead” on Monday. “So he’s actually suffered some military penalties because of what he’s been forced to do because by our threat of action.”

A senior Pentagon official told CNN that during Tuesday’s Foreign Relations hearing, Dempsey will focus on answering questions from senators about the use of military force, rather than making an overt case for military action. It will mainly be Hagel who makes the case that the use of force is warranted, a second senior defense official said. Both declined to be identified because they were speaking in advance of the hearing.

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.

More support for the opposition

U.S. plans for strikes against Syria may be coupled with increased support for rebel forces in that country’s civil war, two leading Republican senators said after meeting with the president on Monday.

Sen. John McCain, the ranking Republican on the Senate Armed Services Committee, and fellow Armed Services member Lindsey Graham said the United States needs to help the rebels reverse battlefield gains by troops loyal to al-Assad.

“We still have significant concerns, but we believe there is in formulation a strategy to upgrade the capabilities of the Free Syrian Army and to degrade the capabilities of Bashar al-Assad,” McCain said.

McCain, who has called for U.S. intervention in Syria since early 2012, criticized Obama’s decision to seek a vote before striking. But he said it would be “catastrophic” for Congress to reject the president’s call to authorize military force.

And Graham told CNN’s “New Day” on Tuesday that without the right American strategy, “the whole region’s going to go down in flames.” Graham, R-South Carolina, said Iran will be emboldened to continue its pursuit of nuclear weapons if no international action is taken against Syria, and the region will be further destabilized.

“Let’s see if we can come up with a strategy that has a chance of working — a military strike to degrade Assad, upgrading the rebel opposition forces, regional players help carrying some of the burden makes sense to me,” he said.

Al-Assad told the French newspaper Le Figaro that an attack on his country risks a regional war, however.

“The Middle East is a powder keg, and the fire is approaching today,” he told Le Figaro in an interview published Monday.

“One must not speak only of the Syrian response, but rather what could be produced after the first strike,” he said. “Because nobody can know what will happen.”

Neither Washington nor Paris has “a single proof” that the Syrian government was behind the alleged chemical weapons attack against civilians on August 21, he said.

Obama would have gone back to the U.N. Security Council if he were a strong leader, Assad said, but instead he has given in to pressure to act from within the United States.

U.N. inspectors await test results

U.N. weapons inspectors left Syria on Saturday with samples that will help determine whether chemical weapons were used in the August 21 attack. Those samples will all be at laboratories by Wednesday and will be tested “strictly according to internationally recognized standards,” Ban told reporters at U.N. headquarters.

Those tests could take up to three weeks — and even then, those tests will only determine whether a gas attack took place, not who was behind it.

“If confirmed, the use of chemical weapons by anyone, under any circumstances, will be a serious violation of international law and an outrageous war crime,” Ban said.

Kerry said Sunday that hat blood and hair samples taken from medics at the scene of the alleged attack point to the nerve agent sarin. The samples reached U.S. hands “through an appropriate chain of custody, from east Damascus, from first responders,” he said.

But David Kay, the former head of the U.S. weapons hunt in Iraq, said the “shadowing effect” of that war makes the U.N. inspectors’ jobs more difficult.

“Remember, these results will be analyzed and re-analyzed around the world,” said Kay, the former U.N. weapons inspector who ultimately determined Iraq had dismantled the chemical, nuclear and biological weapons programs used to justify the American-led invasion in 2003. “So as an inspector, you want to get it right rather than necessarily get it quick”

Kay told CNN the question has been settled for the Obama administration “and for many Americans, including myself. But that’s not enough, because of Iraq.”