Greece's capacity to receive people and assess asylum claims must be strengthened for the deal between the EU and Turkey to work, the United Nations has said.
It also said Turkey must give a fair and timely review to those needing protection.
In a statement, the UN High Commissioner for Refugees (UNHCR) said applicants in Greece should have "the right to appeal before any readmission to Turkey".
Reception and other arrangements need to be readied in Turkey before anyone is returned from Greece.
People determined to be needing international protection need to be able to enjoy asylum, without discrimination, in accordance with accepted international standards.
How this plan is to be implemented is thus going to be crucial. Ultimately, the response must be about addressing the compelling needs of individuals fleeing war and persecution.
The agreement involves closing the EU border on March 20 and sending back migrants who arrive in Greece after that date to Turkey.
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