UNHCR spokesperson Melissa Fleming said the agency was discussing the likelihood with various countries although she would not name them or suggest any possible route.
Read: UN voices deep disappointment at lack of EU consensus on migrants
"We're definitely in touch with different countries on contingencies and UNHCR is ready to move and assist different countries as best we can," Fleming said. "It's going to be just as much a struggle as it has been for Macedonia and Greece."
At least 200,000 migrants have crossed Hungary so far this year, streaming north through the Balkan peninsula having hit Greek shores by various types of boat from Turkey. But two decades of frontier-free travel across Europe have unravelled in the face of an unprecedented influx of people seeking refuge from war and poverty, and Hungary has effectively sealed this entrance to the rest of the EU.
Read: Migrant boat heading to Greece from Turkey sinks, 13 dead
UNHCR expected the numbers arriving at Hungary's border to go down, but the overall flow to continue until around November when the weather worsens, Fleming said. Word of Hungary's border restrictions will have spread instantaneously thanks to social media, she added, so people on the road from Syria will already be making alternative plans.
Serbia, where thousands of refugees are currently backed up trying to enter Hungary, is not a member of the EU. But Serbia borders Romania and Croatia, which are members. Croatia also borders Slovenia, which leads to Italy and Austria.
Read: Refugee Mario highlights migrant crisis one jump at a time
"Maybe we'll try Croatia, then Slovenia and from there to Vienna and Germany," said Emad, a refugee from the Syrian capital Damascus as he entered Macedonia from Greece. "I don't know if it's a good plan, but we have to try."
Croatia has 6,000 police monitoring the state borders and capacity is for some 3,000 refugees, said Zlatko Sokolar, an assistant to the Croatia's chief police director. It plans to register all migrants who come to Croatia, as well as providing initial assistance, but it expects most to see Croatia as a transit country on their way north, Sokolar said.
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