The United Nations' refugee agency has warned it will be watching closely to see how the Turkey-EU migrant deal is implemented, stressing the right to asylum must be paramount.
Reacting after the deal was struck at a Brussels summit, the UN High Commissioner for Refugees (UNHCR) acknowledged the "shared need of countries to find properly managed solutions to this situation."
"We now need to see how this will be worked out in practice, in keeping with the safeguards set out in the agreement - many of which at present are not in place," it said in a statement.
"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. Refugees need protection, not rejection."
Yesterday a summit of European Union leaders and Turkish premier Ahmet Davutoglu approved a controversial deal to curb the huge flow of asylum seekers to Europe, with all migrants arriving in Greece from Sunday to be sent back.
The deal makes Turkey Europe's bulwark against its biggest migration crisis since World War II, but comes at a heavy price and amid criticism from rights groups.
The UNHCR was cautious.
"Importantly, it is explicit that any modalities of implementation of the agreement will respect international and European law," it said.
"In UNHCR's understanding ... this means that people seeking international protection will have an individual interview on whether their claim can be assessed in Greece, and the right to appeal before any readmission to Turkey."
A key immediate priority will be rapidly bolstering Greece's reception conditions and its systems for assessing asylum claims and dealing with people accepted as refugees.
"The safeguards in the agreement have to be established and implemented. This will be an enormous challenge needing urgent addressing," it said.