LONDON — As key nations tighten their borders, thousands of migrants and asylum seekers hoping to enter Western Europe are now bottled up in the Balkans, placing precarious new burdens on a region of lingering sectarian divisions that is exceptionally ill prepared to handle the crisis that has been shunted to it.

More than 17,000 migrants have entered Croatia since Wednesday, and were essentially trapped there, having been blocked from Hungary, sent packing from Serbia and unable to move on to Slovenia. The migrants have become a sloshing tide of humanity, left to flow wherever the region’s conflicting and constantly changing border controls channel them.

Along the roads of eastern Croatia on Friday, the migrants’ detritus — abandoned blankets, torn clothing, empty cans of tuna — littered the highways. On the side of a road outside the border town of Tovarnik, Croatia, three young Iraqi men said they had been stranded for two excruciating days.