Iran says that it warned the United States directly, in mid- and late- 2012, and at least once after that, about the risks of chemical weapons among the rebels. The letter references messages from July 18 and Dec. 1, 2012.
According to the English translation that accompanies the one-page Persian document, the letter reads: “Alerting [worrying] news has been published about the preparations of insurgent forces in Syria for using chemical weapons/elements."
Iran “holds responsible, in addition to the elements of violent forces, their supporter countries including the American government, for any resort to chemical weapons/elements by those insurgent forces,” it states.
The letter makes no reference to the possibility of chemical weapons use by Syria itself.
http://www.csmonitor.com/World/Middle-East/2013/0909/Leaked-Iranian-letter-warned-US-that-Syrian-rebels-have-chemical-weapons