The mass expulsion of migrants from Greece to Turkey under a draft EU-Ankara deal to ease the migrant crisis would be "illegal," the UN warned Thursday, as some of the bloc's own ministers also criticised the plan.
While EU countries squabbled over how to cope with the continent's worst migration crisis since World War II, UN rights chief Zeid Ra'ad Al Hussein said the proposed deal with Turkey raised "a number of very serious concerns".
"Among my concerns is the potential for collective and arbitrary expulsions, which are illegal," he told the UN Human Rights Council in Geneva.
EU leaders had hoped the mooted deal with Turkey -- due to be finalised at an EU summit on March 17-18 -- could stem the flood of migrants streaming through the bloc in search of a better life, many fleeing the war in Syria.
Underscoring yet again the human tragedy of the crisis, a fresh shipwreck off the Turkish coast claimed five lives including a baby as a boat full of migrants heading for Greece capsized.
The EU-Turkey plan drawn up on Monday would see Ankara take back all migrants landing in Greece, in a bid to reduce their incentive to get to Europe.
In return for every Syrian expelled from Greece, the EU would resettle one Syrian refugee from camps in Turkey -- which is hosting about 2.7 million people who have fled the conflict across the border.
In exchange for its cooperation, Turkey wants six billion euros ($6.6 billion) in aid, visa-free access to Europe's passport-free Schengen zone and a speeding up of its efforts to join the EU.
But some EU ministers have voiced opposition to the plan.
"I am extremely critical," said Austrian Interior Minister Johanna Mikl-Leitner on the sidelines of a ministerial meeting in Brussels.
"I am seriously wondering whether we are taking ourselves and our values seriously or if we are throwing them overboard," she said, in a reference to concerns over human rights violations in Turkey.
The migrant crisis has exposed sharp divisions in the 28-member bloc and the leaders of Greece and Germany blasted Balkan countries for slamming shut their borders.
The EU "has no future if it goes on like that," warned Greek Prime Minister Alexis Tsipras, while German Chancellor Angela Merkel said the closures were "neither sustainable nor lasting."
Their reactions flew in the face of the response of EU President Donald Tusk, who welcomed the Balkan route closure as part of a collective response from the bloc.
The strong words came after Slovenia and Croatia barred entry to transiting migrants from Wednesday and Serbia indicated it would follow suit.
EU member Slovenia said it would allow in only migrants wishing to claim asylum there or those seeking entry "on humanitarian grounds" and in accordance with Schengen rules.
The border closures have created a huge bottleneck on the Greece-Macedonia border.
European Migration Commissioner Dimitris Avramopoulos on Thursday urged member states to speed up efforts to help relieve the burden, urging them to admit "at least 6,000" refugees a month from overstretched Greece and Italy.
Under a controversial scheme adopted in September, some 160,000 refugees are to be shared out across the bloc, but only 885 people have been relocated so far.
Slovenia on Thursday announced that it would begin accepting refugees from the scheme in April.
Under pressure at home to reduce the influx, Merkel acknowledged that the western Balkan states' border actions "will obviously bring us fewer refugees, but they put Greece in a very difficult situation".
"If we do not manage to reach a deal with Turkey, then Greece cannot bear the burden for long," she told public radio MDR.
Her Vice Chancellor Sigmar Gabriel noted that while some at home are "secretly pleased that the Balkan states... are doing Germany's dirty work", their actions would not help in the long term.
Merkel is battling to avoid leaving Greece in the lurch as the number of migrants stranded there is still steadily growing.
Meanwhile, Italy's interior minister Angelino Alfano voiced concern that migrants trapped in Greece would seek out alternative routes, such as travelling by sea to Albania and then to southern Italy.
But he added: "For now, there is no sign of such an enormous influx."
Spain's interior minister Jorge Fernandez Diaz worried that migrants might head to north Africa in order to reach Europe through Spain.
"We must not lower our guard," he warned.
Greek authorities said Thursday there were 41,973 asylum seekers in the country, including some 12,000 stuck at Idomeni on the closed Macedonian border.
Zeid said more than 400 people had died trying to reach Europe in the first two months of the year alone.