European Union officials have warned that many hurdles remain ahead of a crucial summit over a tentative deal with Turkey aimed at tackling the massive influx of refugees into Europe.
Leaders of the 28-member bloc are meeting in Brussels on Thursday to agree on an ambitious resettlement plan, which was drawn up at an earlier summit on March 7.
Under the draft deal, Turkey would take back all refugees and migrants who cross into Europe from its soil in return for more money, faster EU membership talks and quicker visa-free travel for Turkish citizens.
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The agreement, however, has faced criticism on several fronts, and European Council President Donald Tusk said there was hard work ahead before it is finalised.
"Work is progressing but there is still a lot to do," Tusk, who chairs Thursday's summit before the arrival of Turkish Prime Minister Ahmet Davutoglu on Friday, said in his invitation letter to EU leaders.
"The catalogue of issues to be resolved before we can conclude an agreement is long."
Potentially the most explosive topic, which diplomats say risks derailing the whole deal, will be how Davutoglu responds to a vague offer to open new "chapters" of Turkey's slow negotiations to join the EU at some distant future date.
Several of these have been blocked by Cyprus over Turkey's refusal to give it the same rights as other EU states in access to Turkish ports and airports.
The island was split into Turkish Cypriot north and an internationally recognised Greek Cypriot south in 1974 when Turkish troops invaded a third of the island after a coup by supporters of union with Greece.
Cypriot President Nicos Anastasiades has made clear he will lift his veto on Turkish accession chapters only if Ankara ends its refusal to recognise Cyprus. Turkey wants concessions for its allies in self-declared Northern Cyprus.
Tusk, who visited Ankara and the Cypriot capital Nicosia this week, said in a draft on Wednesday that the EU would work with Turkey to "prepare for a decision" on opening new accession chapters "as soon as possible" - a vague prospect Davutoglu may not appreciate.
The European Council president has also drafted a set of principles to overcome legal and political objections to the deal, including an assurance against blanket mass returns of refugees.
Under the key clause, in exchange for Turkey taking back all refugees arriving in Greece, the EU will resettle one Syrian for every Syrian allowed to remain on Turkish soil.
EU officials argue this is needed to undercut refugee smuggling networks, but the UN and rights groups have warned that this amounts to the illegal mass expulsion of asylum seekers.
Vincent Cochetel, UNHCR's Europe regional director, said last week that "the collective expulsion of foreigners is prohibited under the European Convention of Human Rights".
He added: "An agreement that would be tantamount to a blanket return of any foreigners to a third country, is not consistent with European law, is not consistent with international law."
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In his letter, Tusk said that "an absolute priority is to ensure that our decisions respect both EU and international law".
His draft to EU leaders said that the one-for-one plan is a "temporary and extraordinary measure which is necessary to end the human suffering and restore public order".
EU officials argue that the alternative to holding people back in Turkey is to see a further build-up of refugees stranded in deteriorating conditions in Greece, whose European neighbours have closed their borders - already over 40,000 people are marooned in Greece.
The Brussels summit comes as the UN said arrivals in Greece alone had topped one million since January 2015, accounting for most of the 1.2 million people who sought asylum in the EU last year as they fled war in Syria, Iraq, Afghanistan and elsewhere.