The Latest on European efforts to respond to massive migration (all times local):

10:45 a.m.

Greece is insisting that European Union leaders meeting in Brussels must provide for sanctions against member states that unilaterally decide to shut out refugees.

Deputy Defense Minister Dimitris Vitsas, who heads a task force on migration, said Thursday that Europe should not contain "fortress-states."

Athens has repeatedly criticized fellow EU member Austria for capping the number of migrants it lets in, which had a domino effect through the Balkans and left nearly 46,000 migrants stuck in Greece.

That figure includes 14,000 living in a waterlogged tent city set up round the closed Idomeni border crossing with non EU member Macedonia.

Greek authorities are trying to persuade people in Idomeni to move to organized shelters elsewhere, but have ruled out using violence to evacuate the camp.

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10:40 a.m.

The top European Union official trying to broker a contested agreement with Turkey to send back tens of thousands warns that the talks will not be easy.

EU Council President Donald Tusk said Thursday that "I am cautiously optimistic, but frankly more cautious than optimistic."

He says any deal at a two-day meeting of EU leaders in Brussels on Thursday must satisfy all the bloc's 28 member countries.

Cyprus is threating to veto a deal because Turkey does not recognize it. Spain rejects any blanket return of migrants. Hungary refuses to resettle refugees from Turkey, saying that would only attract more people to Europe.

The summit is due open in Brussels at 1500 GMT.

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European Union leaders will push ahead Thursday with contested plans to send tens of thousands of migrants back to Turkey amid deep divisions over how to manage Europe's biggest refugee emergency in decades.

With European unity fraying in the face of more than 1 million migrant arrivals over the last year, Turkey — the source of most refugees heading to Greece — is seen as the key partner to contain the influx.

The U.N. refugee agency has reservations about asylum standards in Turkey and rights groups are concerned over Ankara's crackdown on the media and its bloody conflict with Kurdish rebels.

The EU, however, feels it has no better option.

"How are you going to help Greece without having an agreement with Turkey to handle the issue? Do you really want to condemn Greece to become a refugee camp for the rest of Europe?" EU Commission vice president Frans Timmermans said, on the eve of the two-day summit in Brussels.