The EU and Turkey on Thursday night hardened their positions over the EU’s latest migration plan, leaving many disputes still to be resolved on a big-bang proposal to systematically turn back migrants reaching Greek islands.

After five-hours of summit talks in Brussels, EU leaders agreed a negotiating position that further diverged from Ankara’s demands, setting the stage for a clash on Friday over key legal, practical and political elements of the package.

These include Turkey’s refusal to apply full international standards on refugees, a Cypriot veto on opening parts of Ankara’s EU-membership talks, and Greece’s demands for thousands more staff to implement the plan.

Differences also remain on the EU side over promises to accept Syrian migrants directly from Turkey to compensate for those turned back from Greek islands. Angela Merkel, the German chancellor, said the final round of negotiations “will be anything but easy”. François Hollande, the French president, added: “I can’t guarantee you a happy conclusion.”

Talks with Turkey on the offer will begin on Friday morning, when Ahmet Davutoglu, Turkish prime minister, will meet Donald Tusk, the European Council president, Mark Rutte, the Dutch premier, and Jean-Claude Juncker, the European Commission president.

Diplomats said one of the biggest stumbling blocks was the legal status of the plan, with Spain, Portugal and France pressing for Turkey to revamp its asylum system so migrants from all countries are able to seek international protection there.

According to a draft of the EU’s position seen by the Financial Times, the bloc calls on Turkey to offer a “commitment that migrants returned to Turkey will be protected in accordance with the relevant international standards”. Ankara is resisting any such formal legal changes, which EU officials fear would blow a hole in the legal basis for the deal.

Ms Merkel said there were “certain legal prerequisites” needed to ensure migrants would be treated humanely in Turkey. Mariano Rajoy, Spain’s premier, said: “I defended the principle that whatever decision that is adopted must be in conformity with international law, and I made clear that without that we couldn’t support the conclusions.”

Also sensitive are Ankara’s demands that Cyprus lifts a freeze on several “chapters” in its EU membership talks, a concession Nicosia is unwilling to make without Turkey recognising its government. While the issue touches on 40 years of enmity and has the potential to flare up, diplomats are discussing various compromises to work around the Cypriot block. Mr Hollande said leaders were “careful not to name the [specific] chapters”.

On the practical front, Greece also requested 4,000 extra staff — including 2,500 from other EU countries — to help it man borders and detain and handle an estimated 10,000 arrivals per week, who must be processed individually according to the terms of the deal.

Ms Merkel said Germany was willing to contribute personnel to the Greek effort, saying EU member states could be able to commit specific numbers in a matter of days.

“Each and every refugee has to be evaluated, each and every individual case has to be analysed,” Ms Merkel said. “From a logistical point of view, it’s going to be very important to have the necessary personnel on the islands to make this procedure possible.”

Charles Michel, the Belgian prime minister, said: “Even when we have an agreement in the hours to come, we need to be aware that there will need to be enormous logistical [and] operational work to implement it.” He added that staff would be needed so Greece could deal with cases speedily “not in several weeks or months as is the case now”.

Ms Merkel has come under criticism for backing a deal with Turkey that would return thousands of refugees from Greece despite human rights’ groups contention that Turkey is not a safe country for those fleeing the war-torn Middle East.

In the run-up to the deal, hardline officials had called for countries to set an immediate deadline for starting mass returns to Turkey; some say this should be as early as Sunday night. However, leaders were unable to agree on a precise date, with Alexis Tsipras, Greek prime minister, warning a rushed start date would be impossible to implement.

EU leaders agreed that they would resettle up to 72,000 refugees from Turkey under a controversial “one-for-one” scheme, whereby Ankara would be able to send a Syrian refugee directly into Europe for each one it accepts back from Greece. This plan would be “reviewed” once this limit was reached, with an alliance of central and eastern European countries demanding that it cease once this limit is reached.

Turkey will not receive a cast iron promise to reopen new chapters, the legal hurdles a country must clear before it can join the EU. Ankara will instead be offered a promise for a decision on whether to begin these negotiations “as soon as possible”, according to the draft text.

An offer to pay Turkey up to €6bn by the end 2018 was also firmed up by EU heads of government, who promised to “mobilise” the money if commitments from Ankara are met.