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A French anti-terrorism judge says a sarin gas attack in Syria has become a key draw for jihadist fighters who want to battle Bashar Assad's regime in the country's civil war.

Marc Trevidic pointed to "exponential" growth in the number of French residents or citizens who have sought to join jihadists in Syria. French officials estimate the number at around 110.

A U.N. report has confirmed sarin was used in an Aug. 21 attack outside Damascus in which hundreds were killed. The report didn't specify the culprits, but Western powers have accused Assad's regime — a claim he has denied.

Trevidic said Wednesday such suspicion of sarin use by the regime "motivates" foreign jihadists and "demonstrably" has lured more into Syria. He said he didn't have any exact figures.