European Commission Vice President Frans Timmermans also said there would be no "free ride" for Turkey under the deal and that Ankara would need to take agreed steps to win visa-free travel for Turks to the EU.
"There can be no blanket returns," Timmermans told a press conference on the eve of a summit at which Brussels and Ankara hope to seal the agreement.
"We are not turning our back on the refugees because they will be assured adequate international protection, either in the EU or in Turkey."
Asylum seekers also need the "full possibility" of an appeal, he said.
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Under the plan hailed as a "game-changer", Turkey would seek to stop the dangerous sea journeys of refugees and take back all illegal migrants from Greece.
For each Syrian it takes back, it would send another one on to the EU in a more orderly redistribution programme -- a plan Timmermans said would be "temporary and exceptional."
Timmermans meanwhile insisted that the EU was not going easy on Turkey - a long-term candidate for EU membership - on issues such as human rights and visas in order to secure a deal.
"We are certainly not giving Turkey a free ride," he said.