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Brussels • The United Nations and human-rights groups voiced deep concerns Tuesday about the legality of the European Union's plans to send thousands of migrants back to Turkey amid fears the country cannot properly provide for them.

E.U. and Turkish leaders agreed to the broad outlines of a deal that would essentially outsource Europe's refugee emergency. People arriving in Greece having fled war or poverty would be sent back to Turkey unless they apply for asylum.

For every migrant sent back, the E.U. would take in one Syrian refugee, thus trying to prevent the need for people to set out on dangerous sea journeys, often arranged by unscrupulous smugglers.

Turkey stands to gain billions of dollars in refugee aid, faster E.U. membership talks and visa-free travel for its citizens within four months under the plan, whose details are to be worked out at a March 17 E.U. summit.

In another development, Serbia's Interior Ministry said that as of midnight Tuesday, Slovenia will demand valid E.U. visas at its borders, effectively closing the main Balkans migration route to Western Europe for thousands who have continued to cross from Turkey to Greece.

Under the outlines of the proposed deal reached in Brussels, migrants who enter Europe illegally will be sent back and have to join the end of the queue to enter Europe, said German Chancellor Angela Merkel.

But the U.N. and rights groups are not convinced that Turkey is a safe destination. More than 2.7 million refugees, many from Syria, are in Turkey. Most are housed by Turkish families or live out in the open, and few have government-funded shelters.

"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," U.N. High Commissioner for Refugees Filippo Grandi told E.U. lawmakers.

In Geneva, UNHCR Europe bureau Director Vincent Cochetel told reporters that collective expulsion of foreigners is prohibited under international law.

Amnesty International also warned that the plan is legally flawed. Europe's attempt to have Turkey designated as a safe country is "alarmingly shortsighted and inhumane," the group said.

"Turkey has forcibly returned refugees to Syria, and many refugees in the country live in desperate conditions without adequate housing," said Iverna McGowan, head of Amnesty's European office.

"By no stretch of imagination can Turkey be considered a 'safe third country' that the E.U. can cozily outsource its obligations to," she said.

The medical aid group Doctors Without Borders said the deal is cynical and a sign that "European leaders have completely lost track of reality."

"Clearly, Europe is willing to do anything, including compromising essential human rights and refugee-law principles, to stem the flow of refugees and migrants," said the group's humanitarian adviser Aurelie Ponthieu.

U.N. Secretary-General Ban Ki-moon said he's worried by Europe's increasingly tough asylum policies, growing anti-refugee rhetoric and attacks on migrants.

"Extreme right-wing and nationalistic political parties are inflaming the situation where we need to be seeking solutions, harmonious solutions based on shared responsibilities," Ban said.

He added that the EU "can do much more" to manage the influx, which pales next to what Turkey, Lebanon and Jordan have done to take in more than 5 million people.

Europe has been overwhelmed by the arrival of more than 1 million people in 2015 and more than 140,000 so far this year, mostly into Greece via Turkey. In response, nations along the migrant route have built barriers and tightened border controls.