(CNN) -- 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 President Barack Obama on Monday.
Obama met in the Oval Office with Sens. John McCain, and Sen. Lindsey Graham, both members of the Senate Armed Services Committee, the White House said. After the meeting, McCain and Graham said the United States needs to help the rebels reverse battlefield gains by troops loyal to Syrian President Bashar 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," said McCain, the ranking Republican on the Senate Armed Services Committee.
The United States and several of its leading allies accuse al-Assad's forces of resorting to poison gas attacks against rebel forces and civilians, including an August 21 attack near Damascus the Obama administration says killed more than 1,400 people. 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.
In a statement on the meeting, the White House said Obama "underscored that America is stronger when the president and Congress work together to stand up for our national interests." And the administration stepped up its efforts to win congressional authorization on Monday, with Secretary of State John Kerry telling Democratic lawmakers in a conference call that three Middle Eastern nations have offered the use of their military assets for action against Syria.
Those countries -- Turkey, Saudi Arabia and United Arab Emirates -- are the first to do so, but other countries are expected to join them, Kerry said, according to two people who were on the call. Two senior Arab diplomats said talks with Saudi Arabia and the UAE are preliminary, and no details have been discussed.
Kerry said prospect of military strikes has resulted in about 100 defections from the Syrian military, according to the sources. A total of 127 House Democrats were on the call, a Democratic aide told CNN.
But the tough sell for action was clear in the call. When Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said collateral damage from a strike is expected to be low, some lawmakers questioned how officials could know that.
'Big question' still unanswered, congresswoman says
Rep. Janice Hahn, D-California, told CNN that many of those who have listened to administration briefings have questions that have not been resolved.
"What's going be the cost to American taxpayers? How long are we going to be in there? What would signify we have a mission accomplished in this situation?" Hahn asked. But she said her "big question" was whether there was a way to hold Syria's government accountable for violating the post-World War I taboo against chemical weapons "besides this seemingly unclear military strike that could lead to much more conflict in the Middle East."
Dempsey, Kerry and Defense Secretary Chuck Hagel are all scheduled to appear before the Senate Foreign Relations Committee on Tuesday to press the case for action, a senior State Department official said. Kerry will argue that failure to act "unravels the deterrent impact of the international norm against chemical weapons use," said the official, who spoke on condition of anonymity.
And administration officials will be conducting classified briefings on Syria for Congress nearly every day this week. The president will meet Tuesday morning with House Speaker John Boehner and Minority Leader Nancy Pelosi, congressional aides said, and he'd already planned talks with the leaders of the key national security committees in the House and Senate.
One of those, House Armed Services Chairman Buck McKeon, told CNN the administration will have to overcome "a lot of distrust among the American people" about the intelligence that fingers Syria's government in the August 21 attack.
"There will be a real questioning as to the veracity of the evidence and if this really happened or not," McKeon, R-California, said in an interview with CNN's Barbara Starr. "It will be necessary to explain and prove to the American people, and I think the only person who can really do that is the president of the United States."
No vote is expected until after lawmakers reconvene from recess on September 9. McCain, who has called for U.S. intervention in Syria since early 2012, criticized Obama for seeking a vote before striking -- but said it would be "catastrophic" for Congress to reject Obama's call to authorize military force.
"It would undermine the credibility of the United States of America and the president of the United States," McCain said. "None of us want that."
McCain said he was more supportive of a limited U.S. strike after his meeting with Obama, partly because of the prospect of increased support for the rebels. Both he and Graham added, however, that they needed more detailed assurances that the U.S. strategy would be sufficiently strong and sustainable before they could endorse it to their colleagues.
The main support for Syrian rebels has so far come from the Persian Gulf monarchies of Saudi Arabia and Qatar. U.S. aid has been limited largely to non-lethal assistance such as communications gear and medical equipment.
In July, the Obama administration began supplying small arms and ammunition to the rebels. But American officials have struggled with how to back opposition groups without providing weapons to those linked to Islamic militants, such as the al-Nusra Front -- a group considered the most effective anti-Assad force on the battlefield, but one the United States says has ties to al Qaeda.
"It is my belief that the side we choose must be ready to promote their interests and ours when the balance shifts in their favor," Dempsey wrote in an August 19 letter to Rep. Eliot Engel, D-New York, the ranking Democrat on the House Foreign Affairs Committee. "Today, they are not."
The leader of another rebel faction, the Free Syrian Army, said Monday his organization supported Obama's decision to go to Congress for approval for military action.
"We understand, really, the decision-making mechanism in a democratic country and realize that support for the decision will make it stronger and more effective, said FSA chief of staff Gen. Salim Idris. "We hope it will encourage other friendly countries to participate in the international campaign against the regime."
Idris said the FSA -- a force founded by defectors from al-Assad's military -- doesn't share weapons or information with jihadist groups and said any weapons or ammunition it receives "will go to the right hands -- to the hands of my fighters who are moderate, who are fighting to build a free and democratic Syria for all."
Al-Assad: Middle East 'powder keg' could explode
Al-Assad, meanwhile, warned that a regional war could break out if Syria is attacked.
"The Middle East is a powder keg, and the fire is approaching today," he told French newspaper Le Figaro in an interview Monday.
"One must not speak only of the Syrian response, but rather what could be produced after the first strike. Because nobody can know what will happen. Everyone will lose control of the situation when the powder keg explodes. Chaos and extremism will spread. The risk of a regional war exists."
Syria has repeatedly denied being behind the August 21 attack and accuses rebel fighters of using chemical weapons on government troops. U.N. weapons inspectors left Syria on Saturday with evidence that will determine whether poison gas was used in that attack and tests on those samples are being conducted "as fast as it is possible to do within the scientific constraints," said Martin Nesirky, spokesman for Secretary-General Ban Ki-moon.
The United Nations has said more than 100,000 people -- including many civilians -- have been killed 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.
Kerry told CNN's State of the Union on Sunday that blood and hair samples taken from medics point to the nerve agent sarin. But the inspectors won't determine who used the weapons, and al-Assad told Le Figaro, "We have challenged the United States and France to give a single piece of evidence."
Last month's attack wasn't the first time the use of poison gas has been suspected. In April, the United States said it had evidence sarin was used in Syria on a small scale. In May, a U.N. official said there were strong suspicions that rebel forces used the deadly nerve agent. And in June, France said sarin had been used several times in the war, including at least once by government forces.
A new French report released Monday alleges that government troops used sarin at least twice before, in small-scale attacks near Idlib and Damascus. But the August attack was a "massive and coordinated use of chemical agents against civilians," an attack the rebels aren't capable of conducting, the report states.
"No group belonging to the Syrian insurrection has, at this stage, the ability to store and use these agents, let alone in a proportion similar to that used on the night of August 21, 2013," the report concluded. It estimated the death toll to be at least 281, based on videos that captured the attack.
But al-Assad said neither Obama nor French President Francois Hollande, whose government has also called for action against Syria, have been able to provide solid evidence. And he questioned the logic of carrying out an attack he said injured Syrian soldiers as well.
"The French people are not our enemy, but the policy of their state is hostile to the Syrian people," al-Assad told Le Figaro. "... This hostility will end when the French state changes its policy. There will be repercussions, negative as is well understood, against the French interests."
U.S. seeks partner for Syria strikes
France has said it won't act without the United States as a partner. Britain, which had been just as forceful a voice for military action as the United States, won't take part after the House of Commons last week rebuffed Prime Minister David Cameron's call for British military intervention.
And NATO Secretary-General Anders Fogh Rasmussen said that while the August 21 attacks call for a "firm international response," it won't come from the North Atlantic alliance itself.
NATO is prepared to protect Turkey, a NATO member, if Syria attacks it, Rasmussen said. The alliance has deployed Patriot missiles to the country, he said. But, he added, "I don't foresee any further NATO role in Turkey. It is for individual nations to decide how to react to what has happened in Syria."
Obama said Saturday that he preferred multilateral action but added, "It is not in the national security interest of the United States to ignore clear violations of these kinds of international norms."
The U.N. charter generally doesn't allow countries to attack other nations unless in self-defense or with approval from the U.N. Security Council. The Syrian government asked Ban "to shoulder his responsibilities for preventing any aggression on Syria," the state-run Syrian Arab News Agency reported.
Under U.S. law, Obama doesn't have to get Congress' approval to launch military action. The 1973 War Powers Resolution authorizes a president to initiate an attack as long as he notifies Congress within 48 hours. But internationally, a U.S. strike against Syria could be deemed illegal.
Any call for the Security Council to endorse action against Syria would face a sure veto from Russia and China, both Syria's allies. Chinese foreign affairs spokesman Hong Lei said Monday that Beijing is "gravely concerned that some country may take unilateral military actions."
"We believe that any action taken by the international community should abide by the purposes and principles of the U.N. charter ... so as to avoid complicating the Syrian issue and bringing more disasters to the Middle East region," Hong said.
And Russian Foreign Minister Sergey Lavrov said Moscow is "absolutely" not convinced by the evidence the Americans, British and French have shared so far.
"There are no facts, there's only talk about what we know for certain," Lavrov told the state-run RIA Novosti news agency. "When we ask for more detailed evidence, they say, 'You know, it's all secret, so we can't show you.' That means that there are no such facts."
Russia, which has major trade deals with Syria, is sending a delegation to Washington for "dialogue" with members of Congress, the Kremlin said Monday. When the two sides share "opinions and arguments, then we'll better understand each other," said Valentina Matviyenko, speaker of the Federation Council, the upper house of parliament. "And I hope that the U.S. Congress will take a balanced position" and reject military intervention.
In the region, meanwhile, Yemen's parliament announced its opposition to any outside intervention in Syria on Monday.
CNN's Elise Labott, Evan Perez, Barbara Starr, Dana Bash, David McKenzie, Ashley Killough, Sarah Chiplin, Khushbu Shah and Mohammed Tawfeeq contributed to this report.