"They are leaving the murderer and concentrating on the weapons he was using," he said of Assad. "It is like stabbing somebody with a knife then they take the knife away and he is free."
He spoke on the sidelines of a two-day opposition conference in Istanbul.
The talks were the latest in a rapidly moving series of events following the Aug. 21 gas attack on suburbs in Damascus. The U.S. blames Assad for the use of chemical weapons. Assad denies his government was involved and instead points to the rebels fighting a 2-year-old civil war against it.
President Barack Obama began trying to win support at home and abroad for a punitive military strike on Assad's forces, but put that effort on hold when the Syrian government expressed willingness to turn over weapons to international control.