(CNN) -- A regional war could break out if Syria is attacked, Syrian President Bashar al-Assad warned in an interview published Monday.
"The Middle East is a powder keg, and the fire is approaching today," he told French newspaper Le Figaro.
"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 an August 21 chemical weapons attack that killed hundreds of people.
While a U.N. probe is under way, U.S. Secretary of State John Kerry said Sunday there's "an overwhelming case" that Syria was behind the attack.
Blood and hair samples obtained from first responders through an "appropriate chain of custody" have "tested positive for signatures of sarin" gas, Kerry said on CNN's "State of the Union" on Sunday.
It's unclear exactly how the United States obtained the material independently of the United Nations.
Al-Assad told Le Figaro, "We have challenged the United States and France to give a single piece of evidence." U.S. President Barack Obama and French President Francois Hollande "have been incapable" of providing it, he insisted.
He also questioned "the logic" of carrying out an attack that injured Syrian soldiers as well.
NATO Secretary-General Anders Fogh Rasmussen said Monday that "many facts in this case point to the regime as responsible for the chemical attacks."
He called for a "firm international response."
"It would send a very, I would say, dangerous signal to dictators all over the world if we stand idly by and don't react," he said.
But NATO itself won't take military action against Syria.
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."
Britain, which was just as forceful a voice for military action as the United States, also won't take part after a decision by Parliament.
France is also calling for action against Syria.
Al-Assad warns of 'repercussions'
"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.
France has said it won't act without the United States as a partner.
So the world is looking to the United States, waiting to see whether it will act. And following Obama's last-minute decision to hold off until Congress weighs in, no such action is expected until after lawmakers reconvene from recess on September 9.
Lawmakers are split, worried about whether military strikes could worsen the situation -- partly because the opposition in Syria includes al Qaeda-linked extremists such as the al-Nusra Front.
The August 21 attack killed more than 1,000 people -- perhaps more than 1,400, according to U.S. officials.
"We know that the regime ordered this attack, we know they prepared for it," Kerry said Sunday.
"We know where the rockets came from. We know where they landed. We know the damage that was done afterwards. We've seen the horrific scenes all over the social media, and we have evidence of it in other ways, and we know that the regime tried to cover up afterwards."
Obama: Many want action, 'nobody wants to do it'
"(W)e are looking at the possibility of a limited, narrow act that would help make sure that not only Syria, but others around the world, understand that the international community cares about maintaining this chemical weapons ban and norm," Obama said Saturday.
He added that "part of the challenge that we end up with here is that a lot of people think something should be done, but nobody wants to do it."
Obama stressed his preference for multilateral action, but said "it is not in the national security interest of the United States to ignore clear violations of these kinds of international norms, and the reason is because there are a whole host of international norms that are very important to us."
The United Nations charter generally doesn't allow countries to attack other nations unless in self-defense or with approval from the U.N. Security Council.
But United States, Britain, and France couldn't get support from the United Nations for a strike on Syria, because Syria's allies in the U.N. Security Council -- Russia and China -- are sure to block any U.N. effort.
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.
She insisted there are no "strong arguments" for war.
Russia rejects Kerry's claim that the United States has all the evidence it needs.
"We absolutely were not convinced by that (evidence) that our American partners, as well as the British and the French, showed us," Russian Foreign Minister Sergey Lavrov said, according to the state-run RIA Novosti news agency.
"There are no facts, there's only talk about what we know for certain. 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."
China weighed in Monday as well.
"We are gravely concerned that some country may take unilateral military actions," said Chinese foreign affairs spokesman Hong Lei.
"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."
The United Nations, meanwhile, said evidence that could show whether chemical weapons were used in Syria was being delivered to a lab on Monday. But a U.N. spokesman would not estimate how long it may take to get results. Even when results are released, they won't show who was responsible.
Al-Assad's regime Monday asked U.N. Secretary-General Ban Ki-moon "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.
Five U.S. Navy ships are being positioned in the Red Sea, a U.S. official said Monday.
A second official said the aircraft carrier USS Nimitz is not expected to participate in combat operations over Syria, but the ship is there to establish a greater U.S. military presence in the region.
Reports: Sarin's been used in Syria before
World leaders have said previously that sarin has been used in the Syrian civil war.
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.
In June, France said sarin had been used several times in the war, including at least once by the Syrian regime.
But the August attack was by far the deadliest. "This is such a blatant example, we can't pretend not to see it," said Michael Rubin, a former Pentagon official now with the American Enterprise Institute
No end to the bloodshed
While world leaders grapple with what to do about Syria, the reports of carnage on the ground keep rising.
At least 118 people were killed across Syria on Sunday, including 13 children, the opposition group Local Coordination Committees of Syria said.
At least 20 rebels were killed Monday in an army ambush in Adra, northeast of Damascus, the opposition Syrian Observatory for Human Rights said.
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 two years ago.
U.S. Marines site appeared hacked
The pro-Assad Syrian Electronic Army appeared to have hacked the U.S. Marines recruitment website, marines.com, overnight, and posted a letter urging Marines not to attack Syria.
"Dear US Marines, This is a message written by your brothers in the Syrian Army, who have been fighting Al Qaeda for the last 3 years," the message states. "... Obama is a traitor who wants to put your lives in danger to rescue Al Qaeda insurgents."
The message ends by saying, "You're more than welcome to fight alongside our army rather than against it. Your brothers, the Syrian army soldiers. A message delivered by the SEA."
A Marine Corps official said the site was not compromised, but visitors were redirected.
No confidential or personal information was put at risk, the official said.
The Marine Corps Recruit Command cannot immediately confirm who was responsible for redirecting visitors, the official said.