The Red Cross says that migrants are ill-equipped to survive freezing temperatures and some might not make it through winter.
Dozens of migrants crossing from Bosnia into Croatia say they've been attacked by Croatian police.
They also have to avoid landmines left over from the war of the 1990s.
Migrants call it "The Game" because if they're caught, they have to start their journey again, the aim being to reach the EU.
The Red Cross said on Monday that the migrants are ill-equipped to survive freezing temperatures and some might not make it through winter.
The International Federation of Red Cross and Red Crescent Societies (IFRC) warned that more than 5,000 migrants trapped in Bosnia and Herzegovina desperately needed help with winter setting in.
"We are concerned that, without concerted action, migrants will suffer a completely preventable humanitarian disaster," said the IFRC's regional director for Europe Simon Missiri, in a statement.
"The clock is ticking," he stressed.
TRT World's Iolo Ap Dafydd reports from both sides of the border.