EU interior ministers hold fresh talks on migration on Thursday, seeking to reduce the flow of people through the Balkans and plan for what the bloc has warned is a looming humanitarian crisis.
Ministers from non-EU members Serbia, Macedonia and Turkey will also be in Brussels as the European Union reaches outside the borders of the 28-nation bloc in a desperate attempt to deal with the stream of people.
Ahead of the talks, Greek Prime Minister Alexis Tsipras threatened not to cooperate with future EU agreements on the migrant crisis if the burden was not fairly shared among member states.
Athens is seething over a series of border restrictions along the migrant trail to northern and western Europe that has caused a bottleneck in Greece, the main entry point to Europe.
"Greece will no longer agree to any deal if the burdens and responsibilities are not shared proportionally," Tsipras told the Greek parliament Wednesday, adding: "We will not allow our country to turn into a warehouse of souls."
The talks come a day after Austria warned the EU's future was at stake at a meeting of Balkan states in Vienna from which an angry Greece was excluded, and after Hungary announced a referendum on refugee quotas.
The EU remains deeply divided over how to handle the migrant crisis, especially over recent border closures by several member states that have threatened the passport-free Schengen area.
"We want all the contacts on Thursday to allow us to avoid surprises -- we have to avoid that one country is surprised by the measures taken by another," said a source from the current Netherlands presidency of the EU.
Austria in particular has caused anger in the EU by announcing that it will cap asylum applications at 80 a day, saying it is overwhelmed by the numbers coming up through the Balkans from Greece.
Thursday's Brussels meetings will start with a working breakfast between the ministers from Serbia, Macedonia and the EU countries on the main Western Balkans route for migrants and refugees to reach northern Europe.
The EU issued a stern warning on Tuesday that a "humanitarian crisis" was looming, especially in Greece, and that it was "concerned" by developments on the Balkans route.
Brussels was also "coordinating a contingency planning effort, to offer support in case of a humanitarian crisis both outside and within the EU", it said.
The crisis ratcheted up at the weekend after Macedonia closed its Greek border to Afghans because countries further up the route were turning back groups from that country.
Greece is the arrival point for around four fifths of the huge flow of people fleeing war and poverty who are arriving in Europe -- mainly via Turkey -- in the worst crisis of its kind to face Europe for more than half a century.
One hundred thousand migrants have already arrived this year and more than a million last year, according to the International Organization for Migration (IOM).
Greece has been threatened with effective suspension from the frontier-free Schengen zone if it does not do more to stop waving through migrants to other countries, and improve reception and registration conditions for refugees who land on its soil.
The EU's Dublin regulations say that migrants must apply for asylum in the first country they land in but this system is set for an overhaul in the coming months as it has proved unworkable.
Turkey's deputy interior minister will meet ministers from all 28 EU states at the main talks later on Thursday, as the EU pushes Ankara on a deal aimed at cutting the number of arrivals.
Turkey and the EU signed a deal in November under which Ankara agreed to curb the number of refugees crossing to Greece in return for three billion euros ($3.2 billion) in aid and the speeding up of its EU membership bid.
An EU-Turkey summit on the deal will take place on March 7.