The closing of Hungary’s borders has raised concerns among humanitarian groups that migrants seeking to get to Croatia could inadvertently cross through areas near the Hungarian-Croatian border that are littered with thousands of land mines left from the Balkan wars of the 1990s. On Wednesday, Croatian demining experts were sent to the area where many migrants were arriving, Reuters reported.

The countries of the former Yugoslavia, which were torn apart by the wars, have thus far taken a tolerant and welcoming stance toward the migrants, who have viewed the region as a transit zone rather than a final destination. But with Hungary’s decision to criminalize the breaching of its borders, countries like Serbia and Croatia, which are relatively homogeneous and poor compared with some of their richer European neighbors, could soon confront a stream of migrants for which they are ill prepared.

In northern Serbia, a short distance from the border with Hungary, hundreds of people spent the night outside or in temporary tents and expressed determination to enter Hungary, notwithstanding the new ban. Hundreds chanted “Open the door!” and several began hunger strikes.