"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."