He allegedly entered Greece posing as a Syrian refugee.
Migrants began streaming into Slovenia last Friday, when Hungary closed its border with Croatia.
The migration crisis is fanning old tensions in the Balkans, prompting a war of words in a region where bloody memories run deep.
Serbia's press has branded the Croatian PM an "idiot" and a "madman."
Germany currently has 94,000 applications for asylum from citizens of "safe" Balkan countries.
Britain and Germany have put forward a plan to revive Bosnia's stalled bid to join the EU in exchange for commitments from the country to institutional reform