There's no clear evidence such weapons were used at this point.
Were we to get involved, it should not consist of backing any particular armed faction. None of them are people we'd particularly want to be running a country--they are all Islamists of one flavor or another.
At most, we could help stem the humanitarian crisis, but we have a very poor track record of choosing sides to pick winners and losers. And getting involved in other people's civil wars is just a bad idea in general.