European Union leaders will try to convince Turkey's prime minister to help end Europe's migration crisis in return for financial and political concessions but they remain unsure if Friday's (local time) Brussels summit can clinch a deal.
"Tomorrow's negotiations with Turkey won't be very easy," said German Chancellor Angela Merkel, who devised the outlines of the plan with Turkish Prime Minister Ahmet Davutoglu, after EU leaders agreed a common stance on Thursday.
A senior Turkish official told Reuters that Mr Davutoglu would press the EU to open up new areas of negotiation on its long-stalled bid to join the bloc, despite a veto threat by Cyprus.
"We're on the right track but we're not there yet," French President Francois Hollande told reporters after the first day of talks in Brussels. "I can't guarantee you a happy ending."
Thursday's EU discussions revealed considerable doubts among the Europeans themselves over whether a deal could be made either legal in international law, or workable.
Leaders gave EU negotiators a mandate to conclude an accord with Turkey by which it would take back all migrants who reach Greek islands off its coast. In return the EU would take in Syrian refugees direct from Turkey, increase aid for Syrians there, speed Ankara's EU membership process and a scheme to let Turks visit Europe without visas.
Much of the debate, Merkel said, focused on ensuring that a plan that had outraged human rights agencies could ensure that those returned to Turkey, a country with a patchy and worsening record on the matter, would have rights to asylum protected.
"An agreement with Turkey cannot be a blank cheque," Belgian Prime Minister Charles Michel warned, echoing many colleagues who face complaints that Europe is selling out to anti-immigrant nationalists at home by outsourcing its problems to the Turks.
Summit chairman Donald Tusk will open negotiations with Mr Davutoglu at 8.30am (1830 AEDT) on Friday.
A major problem is Turkey's four-decade-old dispute with EU member Cyprus, whose President Nicos Anastasiades insisted there could be no opening of new "chapters" in Turkey's EU membership talks until Ankara allows Cypriot traffic to its sea and airports - a result of a refusal to recognise the Cypriot state.
After EU leaders told Mr Tusk where they could give ground and where they had "red lines", Mr Anastasiades said he was ready to veto a deal if necessary.
There is anger in Nicosia at Merkel for appearing to make Mr Davutoglu an offer last week without having consulted Cyprus at a time when talks on reunification with the Turkish-backed north of the island are at a delicately hopeful stage. Tusk, a former Polish premier, made clear Cypriot interests must be respected.
But Turkey is impatient with what it has called Cypriot "caprice" and a Turkish official in Brussels with Mr Davutoglu said: "The EU has to see the big picture ... We think there are many steps to be taken for the opening of those (accession) chapters. And that is still our expectation."
After a year in which more than a million people have arrived in Europe fleeing war and poverty in the Middle East and beyond, EU countries are looking desperately to Turkey to seal its coastline and stem the flow.