Migrants wade across a river near the Greek-Macedonian border, west of the the village of Idomeni, Greece, March 14, 2016. [Photo/Agencies]

BRUSSELS - 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 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 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."

Even if they can overcome possible Turkish objections, 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.

Over dinner, 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 has 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.

Outside the summit, rights group Amnesty International had planted a large screen in the middle of Brussels' European quarter proclaiming "Don't trade refugees. Stop the deal."

Summit chairman Donald Tusk will open negotiations with Davutoglu at 8:30 a.m. (0730 GMT) to prepare for a lunch meeting at which all 28 EU national leaders aim to wrap up a deal with the Turkish premier.

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 Tusk where they could give ground and where they had "red lines", Anastasiades he was ready to veto a deal if necessary.

There is anger in Nicosia at Merkel for appearing to make 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 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."