(CNN) What do you do with 400 of the world's most unwanted extremists, held in a territory that isn't technically a state, that doesn't really have the legal means to extradite them, or the resources to jail them indefinitely?

That's the question facing Syrian Kurdish forces and their US allies in northern Syria now.

The final stages of the anti-ISIS battle swept up a large number of ISIS fighters, including many foreigners. Some may have been allowed to leave Raqqa, the so-called capital of the caliphate, in the final deal agreed between the Syrian Kurds and ISIS to reduce civilian casualties, under which dozens of ISIS fighters, foreign and Syrian, fled with civilians into the desert.

Other ISIS fighters have been on the run longer. Some are unknown players, but some are also noted criminals, like the so-called "Beatles" -- British ISIS fighters who taunted western audiences as they tortured and executed bound, unarmed hostages kneeling before them.

Their fate has presented the US -- to whom much of the world looks for a solution to this problem -- with a quandary.

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