U.N. Secretary-General Ban Ki-moon said Monday he was "with a heavy heart" submitting a report to the Security Council about the Aug. 21 attack on the Ghouta suburb of Damascus.
U.S. Secretary of State John Kerry said the Syrian government ordered an attack that left more than 1,000 civilians dead. Ban said weapons inspectors were able to confirm chemical weapons were used "on a relatively large scale."
The mandate for U.N. weapons inspectors in Syria was to determine whether chemical weapons were used in the attack, not to say who was to blame.
Ban quoted a report from U.N. weapons inspectors that said "surface-to-surface rockets" were used to deploy the nerve agent sarin in the August attack.
"The United Nations Mission has now confirmed, unequivocally and objectively, that chemical weapons have been used in Syria," Ban said in a statement. "The international community has a responsibility to hold the perpetrators accountable and to ensure that chemical weapons never re-emerge as an instrument of warfare."
The U.S. and Russian governments agreed Saturday to a proposal that would lead to the destruction of Syria's chemical weapons arsenal.
British Foreign Secretary William Hague said he joined his American and French counterparts in supporting a U.N. measure that would pressure Syria to hand over its chemical weapons within a specific time frame.
"The world must be prepared to hold them to account if they don't," he said.
Ban said the use of chemical weapons was a war crime.