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.