A plan by the European Union and Turkey aimed at tackling migration flows came under fierce criticism from human rights advocates on Tuesday, even as thousands of asylum seekers remained stranded in Greece.

Europe has struggled with an influx of migrants and asylum seekers that brought more than 1 million people to its shores last year, with some 140,000 more following since January. Many are fleeing the war in Syria, but economic migrants have also joined their ranks.

Late Monday, EU leaders and Turkish Prime Minister Ahmet Davutoglu agreed to work on a deal under which all migrants reaching Greece from Turkey would be returned to the country, while the EU would directly resettle as many Syrians from Turkey as Ankara took back from Greece.

The aim is to discourage people from taking illegal migration routes into Europe and convince them to apply for asylum from Turkey instead.

Discussions on the new framework continued Tuesday, when Davutoglu and his Greek counterpart, Alexis Tsipras, met in the Turkish coastal city of Izmir.

"Turkey will take back the refugees," Davutoglu said after the meeting. "But in exchange the EU will take the same number of refugees."

Davutoglu also stipulated that visa requirements for Turks also be lifted for the Schengen area, but no final agreement has been reached with the EU over the Turkish premier's requests.

EU officials hope to hash out details of the plan - which includes many legal uncertainties - in time for their next summit, on March 17-18.

But the proposal - under which Turkey can also expect new EU funding and progress in its long-running bid to join the bloc - was described as inhumane and potentially illegal by human rights organizations.

"EU and Turkish leaders have today sunk to a new low, effectively horse-trading away the rights and dignity of some of the world's most vulnerable people," said Iverna McGowan of Amnesty International.

"I am deeply concerned about any arrangement that would involve the blanket return of anyone from one country to another without spelling out the refugee protection safeguards under international law," added UN High Commissioner for Refugees (UNHCR) Filippo Grandi, in a speech to the European Parliament.

UNHCR's Europe regional chief Vincent Cochetel went further, stating that "collective expulsion is prohibited." Sending back all migrants, paying Turkey more money and easing EU entry policies for Turks would not provide a quick fix to the problem, he added.

After arriving in Greece, most migrants have tried to reach northern Europe via the Balkans. But thousands are now stranded in Greece, after countries further north instituted border restrictions.

On Tuesday evening, Serbia and Slovenia said they would not allow migrants without valid Schengen visas to pass through their borders starting at midnight, media reports said.

"That practically closes the Balkan route," a statement from the Serbian Interior Ministry read.

Serbia will not allow migrants to become trapped on its territory, the ministry said.

It was expected that Croatia, which lies between Slovenia and Serbia, would follow suit.

In the Idomeni refugee camp, on the Macedonian border, heavy rainfall has turned the ground to mud and left tents waterlogged. The camp, which was set up for 2,000 people, now holds more than 13,000 who cannot move on. Many have been waiting for more than three weeks.

The crush of people was also felt further north. A refugee from Iraq told dpa by phone that he and a group of others had been stranded on Macedonia's border with Serbia since Friday morning, apparently as authorities there await a clear signal from the EU about the way ahead for migration policy.

The bloc's leaders fell short Monday of declaring the Western Balkan migration route "closed," as countries on the pathway between Greece and northern Europe had demanded. But their joint statement said that irregular migrant flows along the route "have now come to an end."

But the surge into Greece from the south continued Tuesday, with more migrants and asylum seekers arriving on the mainland by ferry after making the dangerous Aegean Sea crossing from Turkey.

Around 1,000 people were expected to disembark Tuesday in Athens' port of Piraeus and to continue north, toward Idomeni.

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