LONDON: Syria has become the preeminent global incubator for a new generation of militants after radical groups more than doubled their recruitment of foreign fighters to as much as 31,000 over the past 18 months, according to a former British spy chief.

In the chaos of Syria’s civil war, the majority of foreign fighters end up in militant groups like Daesh, which uses an extreme interpretation of Islam to justify attacks on its foes and impose repressive rule in the swathes of Syria and neighboring Iraq that it has captured.

“Daesh has seen success beyond the dreams of other terrorist groups that now appear conventional and even old-fashioned, such as Al-Qaeda,” said Richard Barrett, a former head of global counter-terrorism at Britain’s Secret Intelligence Service, known as MI6.

“The number of foreigners joining Daesh has doubled, despite the effort to prevent them doing so,” Barrett, now senior vice president of the Soufan Group, a New York intelligence consultancy, wrote in a report.

Barrett said the appeal of Daesh could not be resolved with bombs alone.

“We have to find better ways to address the Daesh appeal,” Barrett said by e-mail. “This is not about murder and mayhem and more war, it is about the way we see each other.”

He also referred to the riposte by a London bystander who shouted “You ain’t no Muslim, bruv” at a suspected knife attacker as he was detained by police officers at an underground train station.

“The Leytonstone rebuke ‘You ain’t no Muslim, bruv’ does far more to undermine Daesh than dropping bombs on Raqqa,” said Barrett, who between 2004 and 2013 headed the UN team that monitored terrorists.

Most of the fighters flocking to Syria are Arabs, although militants have attracted fighters from across 86 different countries.