US informed by Iran in July 2012 that Syrian rebels had chemical weapons/ Christian Science Monitor

From the CSM, citing leaked diplomatic correspondence it had acquired:

... According to leaked diplomatic correspondence, Iran has been warning Washington since July 2012 that Sunni rebel fighters have acquired chemical weapons, and called on the US to send “an immediate and serious warning” to rebel groups not to use them. 

In a letter acquired by The Christian Science Monitor that was sent sometime in the spring, Iran told American officials that, as a "supporter" of the rebels, the US would be held responsible for any rebel use of chemical weapons...