This citizen journalism image provided by Aleppo Media Center AMC which has been authenticated based on its contents and other AP reporting, shows members of the free Syrian Army hiding behind scrap metal during an attack against Syrian government forces, in the neighborhood of al-Amerieh in Aleppo, Syria, Sunday, April. 21, 2013. The Syrian opposition called on Hezbollah to withdraw its fighters from the country immediately, as activists said regime troops supported by pro-government gunmen linked to the Lebanese Shiite militant group battled rebels Sunday for control of a string of villages near the Lebanon-Syria border. (AP Photo/Aleppo Media Center AMC)
ABU DHABI, United Arab Emirates (CBS) -- Defense Secretary Chuck Hagel says the Syrian regime has likely used chemical weapons on a "small scale."
Hagel was speaking to reporters in Abu Dhabi. He says the White House has informed members of Congress that, within the last day, U.S. intelligence concluded with "some degree of varying confidence" that Syrian President Bashar Assad's regime has used chemical weapons — specifically sarin gas.
Hagel says, quote, "It violates every convention of warfare."
According to a letter sent to Sen. John McCain obtained by CBS News, the assessment of chemical weapons use in Syria was "based in part on physiological samples."
In response to the letter, McCain said he thought it was "pretty obvious" that a red line has been crossed.
President Barack Obama has said the use of chemical weapons would be a "game-changer" in the U.S. position on intervening in the two-year-old Syrian civil war.
Last month, Syria expert Salman Shaikh, director of the Brookings Institution's Doha Center, told CBSNews.com that the government has been "experimenting with various compounds and mixtures to see how they could use these (chemical agents) in a localized fashion."
Shaikh said his sources, who travel to and from Damascus and maintain contact with both current and former regime officials, are certain that Assad's regime has tried out less lethal, less widely dispersed compounds for months.
© 2013 CBS Interactive Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.
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