The UN has confirmed "unequivocally and objectively" that chemical weapons have been used in Syria.
A UN report says sarin was used in a rocket attack in the Syrian capital, Damascus, last month, although it has not attributed blame. "This is a war crime," Secretary-General Ban Ki-moon said.
US allegations that the government was responsible led to threats of military action and then a US-Russia deal for Syria to make safe its chemical arms.
World powers will now try to hammer out a UN Security Council resolution. Earlier, UN investigators said they were probing 14 alleged chemical attacks in Syria since September 2011.