The UN's top officials on refugees and human rights questioned whether the plan would be legal.
EU president Donald Tusk has warned that hard work lay ahead to finalise a proposed deal with Turkey to end Europe's refugee crisis, after Cyprus threatened to derail it over longstanding disagreements with Ankara.
Tusk held hastily arranged talks in Nicosia in an attempt to win Cyprus's backing for the proposal, which has been hailed as a "game-changer" for countries buckling under the burden of a mass refugee influx. But there has been a growing pushback against the deal, with both France and the Czech Republic warning on Tuesday against attempts by Turkey to "blackmail" Europe.
Cypriot President Nicos Anastasiades said his country would not accept "Turkish demands without (the) implementation of Turkey's long-pending obligations," including Ankara's official recognition of the Cyprus government.
EU and Turkish leaders agreed last week to a tentative proposal that calls for the return to Turkey of all new refugees landing in Greece. For each Syrian refugee returned, the EU agreed to take one from a Turkish camp and resettle them in Europe.
Cyprus has expressed reservations, not least as its longtime adversary Turkey expects the accord to lead to the opening of new chapters in Ankara's EU membership bid and the easing of visa requirements in Europe's passport-free Schengen area.
From Nicosia Tusk flew on to Ankara for talks with Turkish Prime Minister Ahmet Davutoglu, saying afterwards it was "not an easy task" to make the proposal legally sound and acceptable to all 28 EU members. "It is clear that there is still hard work to be done," Tusk said, adding there were "a catalogue of issues" to address if an agreement was to be reached at a new summit on tomorrow and Friday.
The UN's top officials on refugees and human rights questioned whether the plan would be legal and Tusk conceded this was an issue that still had to be worked out. Paris on Tuesday insisted that Turkey will not be allowed to dictate terms at the summit.
France will tell Turkey it wants "more efficient" cooperation on the migrant crisis, but will warn against any attempt at "blackmail", Prime Minister Manuel Valls said. Czech President Milos Zeman also lashed out at Turkey on Tuesday, claiming that Ankara was seeking billions of euros more in EU aid. "Impolite people like myself call that blackmail," he told reporters.
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 "European Council President Donald Tusk addresses the media in Ankara, Turkey, in this March 15, 2016 handout photo")
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