The U.N. refugee agency is warning of a "buildup" of migrants and refugees in Serbia as its neighbors tighten their borders to the influx.

Adrian Edwards of UNHCR says "the crisis is growing and being pushed from one country to another" as roughly 4,000 migrants and refugees pour into Greece each day and head north.

He says stricter border controls first by Hungary and more recently Croatia threaten a bottleneck in Serbia, "which is not a country with a robust asylum system."

Speaking Friday to reporters, Edwards said: "You aren't going to solve these problems by closing borders."

UNHCR says more than 442,440 people have crossed the Mediterranean this year, and 2,921 have died trying. But the International Organization for Migration puts those figures at 473,887 and 2,812, respectively.