A UN report expected next week will "overwhelmingly" confirm that chemical weapons were used in Syria last month, the secretary general says, BBC reported.
Ban Ki-moon made no comment on who was to blame for the 21 August attack in the Ghouta area of Damascus, as that is not part of the report's remit.
But he did say Syria's president was guilty of "crimes against humanity".
The US and Russian foreign ministers are continuing their talks on a plan to make safe Syria's chemical weapons.
A spokeswoman for Russian Foreign Minister Sergei Lavrov said the talks with US Secretary of State John Kerry in Geneva would head into the night, adding: "They are working on some real substance."