According to AP reports, Iran is heavily involved in the conflict between Syria and the opposition forces at this moment. Specifically, they are "building and training a militia to fight against opposition forces." Army General Martin Dempsey, chairman of the Joint Chiefs of Staff, and Sec. of Defense Leon Panetta have both voiced concern over Iran's growing role.
Panetta, in particular, said it's clear that Iran is doing more and more for the Syrian regime, and he's not happy about it: "We do not think that Iran ought to be playing that role at this moment in time. It's dangerous, and it's adding to the killing that's going on in Syria."
The Sec. of Defense also believes Iran's growing presence drags the conflict out further, inasmuch as it "bolsters a regime [the U.S.] thinks ultimately is going to come down."