United Nations secretary general Ban Ki-moon says a United Nations report expected next week will "overwhelmingly" confirm that chemical weapons were used in Syria last month.
His comments come as the United States and Russian foreign ministers discuss securing Syria's chemical stockpile.
Mr Ban said on Friday Mr Assad had committed crimes against his own people.
He was speaking at a women's forum that had barred journalists but his address was broadcast on UN media.
Mr Ban said the Syrian president had committed many crimes against humanity.
He also said a forthcoming report from UN weapons inspectors would overwhelmingly conclude that gas was used against civilians.
"I believe that the report will be an overwhelming report that chemical weapons (were) used, even though I cannot publicly say at this time before I receive this report," he said.
