The United Nations and human rights groups warned on Tuesday 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," U.N. High Commissioner for Refugees Filippo Grandi told the European Parliament in Strasbourg.
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". Relief charity Doctors without Borders said it was cynical and inhumane.
But Davutoglu insisted the preliminary deal would not stop Syrian refugees legitimately seeking shelter in Europe.