On Thursday U.S. Defense Secretary Chuck Hagel said that the administration has evidence "to some degree of varying confidence" that Syria has used chemical weapons, Robert Bruns of the Associated Press reports.
Obama has previously warned Syria against crossing a "red line" on chemical weapons, saying it would be a "game-changer" toward his military approach to the conflict.
"It violates every convention of warfare," Hagel said.
It's unclear what the international response will be, as the West has become increasingly concerned about the increasing power of al-Qaeda-linked Syrian rebels than toppling of Assad. Hagel said that the "red line" issue is a "policy question."
Earlier this month, Western diplomats said they had "hard evidence" chemical weapons had been used at least once by the Syrian army. Britain and France submitted information to the UN about allegations of chemical weapon use in the cities of Homs last December and Damascus last month.
On Dec. 23 ,there were multiple reports the Homs attack, and doctors belonging to Syrian American Medical Society (SAMS) subsequently corroborated the claims when they described a "probable" use of what chemical specialists refer to as Agent-15 (known as BZ to NATO).
This is the first time the White House has confirmed their use.
Here's the full letter the Obama administration sent to Sens. McCain and Levin:
Letter from President Barack Obama to John McCain on Syria chemical weapons