Growing numbers of foreigners are going to Syria to fight alongside the rebels.
"Most of them are going to join the more conservative Islamist elements," says Shiraz Maher.
Maher is senior research fellow at the International Centre for the Study of Radicalization at King's College, London, which recently published a study on the phenomenon.
"If not al-Qaeda, then certainly at the top end of the spectrum, in the more religious sphere," he says.
"These fighters will be heavily indoctrinated with religious zeal, but they're also going to be brutalized by a conflict that is one of the worst we've seen in a long, long time," adds Maher.