The United Nations and human rights groups have warned that a tentative European Union deal to send back all irregular migrants to Turkey in exchange for political and financial rewards could be illegal.
"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," UN High Commissioner for Refugees Filippo Grandi told the European Parliament in Strasbourg on Tuesday.
He was speaking hours after the 28 EU leaders sketched an accord with Turkish Prime Minister Ahmet Davutoglu in Brussels that would grant Ankara more money to keep refugees in Turkey, faster visa-free travel for Turks and a speeding up of Ankara's long-stalled membership talks.
Rights group Amnesty International called the proposed mass return of migrants a "death blow to the right to seek asylum".
But the executive European Commission insisted the deal to put an end to a mass influx of more than a million people fleeing war and poverty in the Middle East and beyond, due to be finalised next week, was fully legal.
German Chancellor Angela Merkel, who pushed for the accord to assuage anxious voters before regional elections on Sunday, said things were finally moving in the right direction after nearly a million Syrians, Iraqis, Afghans and others flooded into Germany alone last year.
She denied accusations that Turkey was using refugees to blackmail Europe.
The 28 EU leaders were taken by surprise by the bold, last-minute Turkish initiative, which went beyond previous plans for more limited cooperation.
Unable to sign up to firm commitments immediately, they agreed to wrap up a deal at their next summit on March 17-18, but several points remain sensitive.
Migrants marooned in squalor on Greece's frontier with Macedonia by the closure of borders further north vowed to keep trying to cross Europe to wealthy Germany, while Syrian refugees in Turkey said they too would not be deterred by the lockdown.
Critics denounced a cascade of border closures down the main Western Balkan migration route that has left 33,000 people stranded in Greece, causing a humanitarian catastrophe.