It has been reported but not verified by Israel that Assad has used nerve gas on civilians

Yesterday US defence secretary Chuck Hagel, who is visiting Israel, said American intelligence agencies were still assessing whether chemical weapons might have been used. He added: “The United States, along with Israel, have options for all contingencies and certainly the Syrian government’s use of chemical weapons would be a game changer in crossing that red line.”

The 1925 Geneva Convention - Prohibits the use of "asphyxiating gas, or any other kind of gas, liquids, substances or similar materials". Syria is signed up to the convention.

I don't think this is enough to get round a Russian/Chinese UN Veto against military intervention?

We have evidence Syria military has used nerve gas, claims Israel - World - News - London Evening Standard

From an international legal perspective would Assad gassing his own people change the rules? Or just create a heavy case for intervention at the security council?