ISIS takes away passports of Foreign Fighters so they cannot leave

The jihadis from other countries tend to stick in groups by nationality or language – the British together, the Dutch together – and they don’t have much to do with ordinary people here. Both sides are afraid of each other.
If someone tries to talk with the foreigners, the Islamic police are likely to turn up and ask why he is bothering them, or perhaps accuse him of being a spy. But the foreigners are also nervous, perhaps because their families or governments in their countries don’t know they are here. Maybe they are worried that their photos or real names might be published, and that this will cause problems if they want to go home.
In fact, though, there is little chance of them going back. It’s easy to get into Raqqa, but very hard to get out. When foreign fighters go to Raqqa the first thing they do is confiscate their passports, and in sermons at the mosques Isis has warned people against giving foreigners new IDs.
Some of them just get bored when they arrive here from London or New York. Raqqa was never an exciting city and now there is nothing to do at all, so they lose enthusiasm. Others just came to live a good life under the caliphate, but they don’t really want to fight, so when Isis needs men and tries to take them to the frontline they are unhappy.
Isis can’t afford news to get out of people defecting, so anyone attempting to sneak out is executed in secret. They killed several people in the west of the city and just dumped their bodies in a hole until the smell got so bad that they had to bury them.
end quote from:
http://www.theguardian.com/world/2015/feb/21/islamic-state-capital-raqqa-syria-isis