EU interior ministers expressed concern Thursday that migrants might seek alternative routes to northern Europe after their passage through the Balkan countries was sealed off, while the arrivals in Greece continued undeterred.

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

The main route for people trying to reach wealthy northern Europe has been from Turkey via Greece, and onward through the Western Balkans. But countries along that route shuttered their borders this week, leaving thousands stranded and creating a bottleneck in Greece.

Italian Interior Minister Angelino Alfano said his country is working with Albania to prevent a new migration route from opening across the Adriatic Sea and flagged the issue to his 27 EU counterparts, at talks in Brussels on Thursday.

"Logic suggests that, if there were an influx from Turkey into the Balkan route and if walls were to interrupt the journey towards northern Europe, this route could open," Alfano said, while noting that there was no evidence of this happening at present.

Without legal pathways into Europe, "we shall see migrants and the smugglers, the ruthless smugglers that are behind them, trying to find new routes," warned EU Migration Commissioner Dimitris Avramopoulos.

German Interior Minister Thomas de Maiziere warned that the Mediterranean Sea crossing between Libya and Italy might again become popular, as it was before Greece became the main conduit to Europe.

Others have raised the prospect of Bulgaria becoming a new transit country. The EU member state shares a land border with Turkey, but is not part of Europe's free-travel Schengen zone.

Despite this week's border closures, migrants are still attempting the dangerous Aegean Sea crossing to Greece.

Five people, including a baby, drowned off Turkey's coast when their boat capsized late Wednesday, the Dogan news agency reported.

In recent days, NATO began monitoring the sea route to help crack down on migrant smugglers. European Commission President Jean-Claude Juncker expressed confidence on Thursday that this will lead to "a substantial reduction of irregular crossings" in the Aegean.

NATO has five vessels - most of them equipped with helicopters - in the area, mainly focused around the Greek island of Lesbos, Secretary General Jens Stoltenberg told journalists after a meeting with Juncker. More ships are due to join the operation, which will soon move south, the NATO chief said.

But the arrivals in Greece have so far remained steady.

"Yesterday, 2,073 people arrived on the islands," a spokesman for the Greek migration crisis centre said Thursday.

An estimated 13,000 people are stranded in a makeshift camp at Idomeni on the Greek border, where downpours have turned the ground into an ocean of mud.

Doctors from the nearby town of Polikastro said that hundreds of children and adults were suffering from respiratory and intestinal infections.

Around 250 people, mostly families, agreed to be relocated to a camp in the Athens area, 550 kilometres to the south.

In Brussels, Avramopoulos warned that the border closures along the Western Balkan route have created a "humanitarian crisis that risks to turn to a humanitarian disaster" in Greece.

The commission announced that it would make available another 275 million euros (299 million dollars) in emergency assistance for asylum and border control operations.

This takes to 464 million euros the aid provided through two EU funds to help "address the most urgent funding needs of member states in the context of the refugee crisis," the EU's executive said.

In a desperate bid to ease the flows into Europe, EU leaders agreed Monday to work with Turkey on a plan under which Ankara would take back any new arrivals to Greece, while the bloc would directly resettle Syrian refugees out of Turkey.

The deal, which is still being finalized, has drawn heavy criticism from rights groups and EU lawmakers. UN high commissioner for human rights Zeid Ra'ad Al Hussein weighed in on Thursday.

"Among my concerns is the potential for collective and arbitrary expulsions, which are illegal," Zeid said.

Returning Syrian refugees to Turkey, Lebanon or Jordan would expose them to unbearable living conditions, he added, urging the EU to adopt a "more rights-compliant and humane set of measures."

Meanwhile, Turkish Minister for European Affairs Volkan Bozkir said that, in total, "maybe tens of thousands of refugees" would be returned from Greece, noting that the scheme would not apply to those already in the country.

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