Budapest, Hungary (BBN)-Hundreds of migrants have spent the night at the closed Hungary-Serbia border, with many facing the choice of finding a new route to northern Europe.
Serbian media said buses were now being redirected to Croatia, with migrants then planning to move on to Slovenia, Austria - and finally Germany, reports BBC.
There have also been suggestions that Romania may become a new route.
New border restrictions and a row over allocating migrants have shown bitter divisions in Europe over the crisis.
Hundreds of stranded migrants spent the night in the open or in makeshift tents close to the Serbian border with Hungary. Some gathered wood for fires.
A day earlier, new Hungarian laws came into effect and police sealed a railway crossing point that had been used by tens of thousands of migrants.
One Syrian, Mohammad Mahayn, told Reuters he had breached the razor wire to let his wife through and they were now separated. "I had hope until now, but it's all gone," he said.
At midnight (22:00 GMT), Austria became the latest country to introduce tighter border controls. They apply to the border with Hungary but could be extended.
But Austria insisted no-one fleeing persecution would be turned back to Hungary.
Austrian police said Vienna's two main stations were overflowing with migrants and the main station in Salzburg might have to close.
Austria's director general for public security, Konrad Kogler, said: "We expect that if [Hungary's] measures are very effective we will have to deal with different, new routes."
The imposition of temporary border controls are a challenge to the EU's Schengen agreement on free movement.
The 28-member bloc is facing a huge influx of migrants, many of whom are fleeing conflict and poverty in countries including Syria, where a civil war has been raging since 2011.
Follow the latest developments on the migrant crisis:
Fergal Keane on the Hungary-Serbia border
Will Hungary's clampdown work?
The refugees who chose Brazil over Europe
Smuggling people into the country thousands want to leave
Five 'happy refugees' pictures that went viral
Afghan boy's lonely start in Germany
What next for Germany's asylum seekers?
Crisis explained in graphics
'We have to try'
In Serbia, buses were being prepared to take migrants to the Croatian border, local media said.
Serbia's B92 and Blic websites said the buses were due to leave the town of Presevo, southern Serbia, for the Croatian border town of Sid.
Some of the refugees confirmed that they planned to try new routes.
"Maybe we'll try Croatia, then Slovenia and there to Vienna and Germany," Syrian migrant Emad, who is currently in Macedonia, told Reuters.
"I don't know if it's a good plan, but we have to try."
Abolfazl Ebrahimi, a 17-year-old Afghan who is in Athens, told Associated Press he also now planned to try the Croatia route.
Croatia has reportedly been preparing for such a scenario, with thousands of police monitoring its eastern borders.
'End of the EU'
The Serbian minister in charge of the government's working committee on migrants, Aleksandar Vulin, argued that the closure of the border by Hungary was unsustainable.
He told the BBC's Lyse Doucet that contact between Serbian and Hungarian officials had been minimal.
"We have some kind of negotiations, if you can say so, with Hungarian counterparts, with a police officer - someone who is in charge, through the fence. And we ask, can we talk somewhere... can we find some place to see each other? They said no. Through the fence."
Hungary earlier warned that anyone who crosses the border illegally would face charges.
Its new laws make it a criminal offence - punishable by prison or deportation - to damage the newly built 4m (13ft) razor-wire fence along Hungary's 175km (110 mile) border with Serbia.
Hungary has also said it could extend its fence to the border with Romania.
Romania said this would violate the "European spirit" of cooperation.
Human rights groups have condemned Hungary's move, with Amnesty International saying Budapest's "intimidating show of militarised force is shocking".
European Migration Commissioner Dimitris Avramopoulos is due to visit Hungary, which has declared a state of emergency in its two southern counties, on Wednesday.
EU members have also clashed over mandatory quotas for refugees.
One German minister called for financial penalties for nations who resisted, sparking accusations from the Czech Republic of "empty threats" and a warning of the "end of the EU" from Slovakia.
The EU's border agency says more than 500,000 migrants have arrived at the EU's borders this year, compared with 280,000 in 2014.
BBN/SK/AD