Syrian President Bashar Assad delivers a speech in Damascus, Syria, on June 3.
PARIS (CBSNews.com) - France released an intelligence report on Monday alleging chemical weapons use by Syria's regime that dovetailed with similar U.S. claims, as President Bashar Assad warned that any military strike against his country would spark an uncontrollable regional war and spread "chaos and extremism."
The verbal crossfire, including a rejection of the Western allegations by longtime Syrian ally Russia, was part of frenzied efforts on both sides to court international public opinion after President Barack Obama said he would seek authorization from Congress before launching any military action against Assad's regime.
"The Middle East is a powder keg, and today the flame is coming very near. We cannot talk merely about the Syrian response, but about what might take place after the first strike. But nobody knows what will happen. Everyone will lose control of the situation when the powder keg explodes. Chaos and extremism will spread. There is a risk of regional war," Assad said in an interview published Monday by French newspaper Le Figaro.
Last week, the U.S. laid out its intelligence assessment, blaming Assad's forces for an Aug. 21 chemical attack outside Damascus that killed, according to the U.S., 1,429 people, including 426 children.
The U.S., with warships at the ready in the Mediterranean, appeared poised to launch missile strikes against Assad forces, but President Obama announced Saturday that he would seek congressional approval for any military action.
On Sunday, Kerry said the U.S. received new physical evidence in the form of blood and hair samples that shows sarin gas was used in the Aug. 21 attack. Kerry said the U.S. must respond with its credibility on the line.
"We know that the regime ordered this attack," he said. "We know they prepared for it. We know where the rockets came from. We know where they landed. We know the damage that was done afterwards."
Mr. Obama has faced resistance to the proposed strike both internationally and domestically, but France has backed the American position. On Monday, the French government released its own intelligence estimate, concluding that the Assad regime launched the attack involving "massive use of chemical agents" and could carry out other strikes of a similar nature in the future. The confirmed death toll cited in the French estimate - based in part from dozens of videos culled by French intelligence services - was at least 281, lower than the American toll.
Assad dismissed the findings from both countries.
"It is for those who are making the accusations to provide the proof. We have challenged the United States and France to put forward a single proof. Obama and Hollande have been unable to do so, even to their own people. ... I'm not at all suggesting that the Syrian army does or does not possess such weapons. Let's suppose that our army wishes to use WMD: is it really going to do so in an area where it is actually present and where soldiers have been wounded by these weapons, as the UN inspectors found during their visit to the hospital where they were being treated? Where is the logic in that?"
U.N. chemical inspectors toured the stricken areas last week, collecting biological and soil samples, but it is not clear when they will present their findings.
The Obama administration has failed to bring together a broad international coalition in support of military action, having so far only secured the support of France.
Britain's parliament narrowly voted against British participation in a military strike last week, despite appeals by Prime Minister David Cameron, and the Arab League has stopped short of endorsing a Western strike against Syria.
In an emergency meeting on Sunday, the 22-state League called on the United Nations and the international community to take "deterrent" measures under international law to stop the Syrian regime's crimes, but could not agree on whether to back U.S. military strikes.
Russia or China would likely veto any U.N. Security Council resolution sanctioning a Western strike against Syria.
China is "highly concerned" about possible unilateral military action against Syria and believes the international community must "avoid complicating the Syrian issue and dragging the Middle East down into further disaster," Foreign Ministry spokesman Hong Lei said Monday.
Russia, which along with Iran has been a staunch supporter of Assad through the conflict, brushed aside Western evidence of an alleged Syrian regime role.
"What our American, British and French partners showed us in the past and have showed just recently is absolutely unconvincing," Russian Foreign Minister Sergey Lavrov said Monday at the country's top diplomatic school. "And when you ask for more detailed proof they say all of this is classified so we cannot show this to you."
Lavrov said "there was nothing specific there, no geographic coordinates, no names, no proof that the tests were carried out by the professionals." He did not describe the tests further.
Russian President Vladimir Putin proposed Monday to send a delegation of Russian lawmakers to the U.S. to discuss the situation in Syria with members of Congress. Two top Russian legislators suggested that to Putin, saying polls have shown little support among Americans for armed intervention in Syria to punish its regime for an alleged chemical weapons attack.
In Washington, the Obama administration was lobbying to secure domestic support.
Mr. Obama met Monday with former political rival Sen. John McCain at the White House, hoping the foreign policy hawk will help sell the idea of U.S. military intervention.
On Capitol Hill, senior administration officials briefed lawmakers in private on Sunday to explain why the U.S. was compelled to act against Assad. Further meetings were planned from Monday to Wednesday.
The Syrian conflict erupted in March 2011 as an uprising against Assad that later degenerated into a civil war. More than 100,000 Syrians have been killed in the conflict.
In Damascus, the Syria representative of the U.N. refugee agency, Tarik Kurdi, said that five million Syrians have been displaced inside the country by the war.
In addition, nearly 2 million Syrians have fled to neighboring countries, according to previous U.N. figures, bringing the total number of uprooted Syrians to about 7 million, or nearly one-third the country's estimated population of 23 million.
Kurdi said the need for aid is far greater than what the international community has provided so far.
"Whatever efforts we have exerted and whatever the U.N. has provided in humanitarian aid, it is only a drop in the sea of humanitarian needs in Syria," he told The Associated Press. The funding gap "is very, very wide," he added.
© 2013 CBS Interactive Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.