Migrants hoping to trek from Greece toward northern Europe found their path blocked on Wednesday after a string of western Balkan nations shut their borders, exacerbating a dire humanitarian situation on the Macedonian frontier.

German Chancellor Angela Merkel warned on Thursday that closing the Balkan route "does not resolve the problem" and that this move is "not sustainable".

"You don't resolve the problem by taking a unilateral decision," Merkel said. "If we do not manage to reach a deal with Turkey, then Greece cannot bear the weight (of migrants) for long".