Austria's new daily and annual limits for asylum requests raised tensions with the European executive body Thursday, when EU Migration Commissioner Dimitris Avramopoulos said the step was unlawful.
Austrian Interior Minister Johanna Mikl-Leitner announced Wednesday that her country would limit the entry of refugees at the border to 3,200 per day for people transiting to other European countries, and to 80 for those who seek asylum in Austria.
In January, the Vienna government had set a limit of 37,500 asylum applications for this year, a fraction of last year's 90,000.
"Such a policy would be plainly incompatible with Austria's obligations under European and international law," Avramopoulos wrote to Mikl-Leitner, in a letter obtained by dpa.
The disagreement between Vienna and Brussels came into the open just as leaders of the 28 EU members states met in Brussels Thursday, where the migration crisis is one of the top items on the agenda.
Most migrants have been making their way from Turkey to Greece and up through the Western Balkans to wealthy northern European countries. The European Union has attempted to get Turkey to hold back migrants, but with limited success so far.
EU leaders meeting in Brussels will call for "further, decisive efforts" from Ankara and "a substantial and sustainable reduction of the number of illegal entries from Turkey into the EU," according to a draft of their statement seen by dpa.
If this does not materialize, "the talks about closing the borders of central Europe and securing the borders of the Western Balkans will become a new reality," Czech Prime Minister Bohuslav Sobotka warned.
German Chancellor Angela Merkel said the priority should be the implementation of a migration deal that the EU and Turkey have struck, despite a push by other leaders to explore further options to stem migrant flows.
Merkel and 10 other EU leaders had been scheduled to meet with Turkish Prime Minister Ahmet Davutoglu ahead of their summit, but his trip was cancelled following a deadly bombing in Ankara. Austrian Chancellor Werner Faymann said he plans to organize a new meeting.
Avramopoulos pointed to the Geneva Convention and the European Convention of Human Rights, among others, and urged Austria to reconsider its policy.
"Austria has a legal obligation to accept any asylum application that is made on its territory or at its border," Avramopoulos added.
He also expressed concern over the quota for people transiting to Germany or other European countries, pointing out that the European Commission was opposed to a "wave-through" approach.
His comments came as the police chiefs of Austria, Slovenia, Croatia, Serbia and Macedonia signed an agreement in Zagreb with immediate effect to screen migrants at the Macedonian border and transport them on to Austria and Germany, Croatian state TV HRT reported.
People who do not pass the initial filter will remain in reception centres, Croatian police director Vlado Dominic said.
Austrian Chancellor Faymann dismissed the criticism of his country's planned daily limit.
"Legal opinions will be answered by legal experts. From a political viewpoint, I say: Our position will not change," he said in Brussels.
However, refugees have been arriving at Austria's southern borders at far lower numbers than 3,200. Police expected only 750 people to arrive from Slovenia on Thursday.
Balkan countries are following Austria's cue for heightened border security with plans to stop migrants at their borders.
Slovenia is in the process of giving the army authority to police borders, while Macedonia began reinforcing and expanding a mesh and razor-wire fence on the path of migrants arriving from Greece.
In Belgrade, the minister in charge of migration, Aleksandar Vulin, said: "Serbia will do as Austria does."
"We will not pass our problems to others, but we'll not allow anyone else's problems to be resolved on Serbian soil," Vulin said.
Closing borders would not stop migrants from taking the trip to Greece and may create a humanitarian emergency in the country, a Greek minister warned.
"The surge will not stop. The refugees seek protection because their lives are in danger. They are desperate," the Greek alternate minister of immigration policy told the German daily Handelsblatt.
Mikl-Leitner defended her policies by pointing to similar daily quotas by neighbouring Germany, which had resulted in up to 18,000 migrants being stranded temporarily in Austria last year.
She also told dpa that Austria's tighter border policies were needed to keep nationalist sentiments at bay in Europe.
"These measures are not against Europe, but for Europe," said the minister, who is under pressure from growing support for the anti-foreigner Freedom Party.