Slovenia has introduced new border restrictions for migrants as part of efforts to close the Balkans route from Greece to Western Europe.
Only migrants who plan to seek asylum in the country, or those with clear humanitarian needs will be allowed entry.
In reaction, Serbia said it would close its borders with Macedonia and Bulgaria to those without valid documents.
The future of the EU’s passport-free Schengen zone is already in doubt.
Eight of its members, including Austria, Hungary and Slovakia, have tightened border controls, leaving thousands of migrants stranded in Greece.
Europe is facing its biggest refugee crisis since World War Two. Last year, more than a million people entered the EU illegally by boat. Most of them were Syrian, fleeing the country’s civil war.