The Syrian president insists that the U.S. and others have not been able to back their claims that Syrian government forces used chemical weapons in an Aug. 21 attack.

A day after President Obama announced he'll wait for congressional authorization before launching strikes on Syria; members of Congress attended a classified briefing at the Capitol. Even though there's still one week left of summer recess, dozens of lawmakers flew to Washington, D.C. from their home districts just for the meeting.

"I can share with you today that blood and hair samples that have come to us through an appropriate chain of custody from east Damascus, from first responders, it has tested positive for signatures of sarin," Secretary of State John Kerry told CNN on Sunday.

About one-quarter of all Syrians have been forced to flee their homes, many to neighboring nations. New data on the number of refugees come as Congress begins debating the president's request for authorization to take military action in response to the Assad regime's alleged use of chemical weapons.