By Tom Krkemeier and Gabriela Baczynska
The European Union has warned that a deal with Turkey to curb mass migration to Europe hinges on Ankara acting to support peace talks in EU member Cyprus.
European Council President Donald Tusk, who will chair an EU summit on Thursday and Friday, said much remained to be done to reach a deal with Turkey.
"Work is progressing but there is still a lot to do," he said in a letter to EU leaders.
The migration deal needed to be "an opportunity (for Turkey) to support the settlement talks in Cyprus. Only if this is possible can we move forward here".
EU officials offered last-minute tweaks to the draft pact with Turkey in an effort to make it legally watertight, but a standoff with Cyprus could yet scupper any deal this week.
Under a tentative agreement reached last week, Ankara would take back all migrants and refugees who enter the EU from its shores or are detained in its territorial waters, in return for more money, faster visa-free travel for Turks and a speeding up of its slow-moving EU membership negotiations.
For its part, the EU would admit one legal Syrian refugee directly from Turkey for each one trying to reach Europe by boat and taken back by Turkey from the Greek islands in a step meant to wreck the business model of people smugglers.
But Cypriot President Nicos Anastasiades has threatened to veto any progress in Turkey's accession talks unless Ankara meets its obligation to open Turkish ports and airports to Cypriot traffic, effectively recognising his state.
"We are certainly not giving Turkey a free ride," European Commission Vice President Frans Timmermans said.
Ankara would have to enact a raft of measures within six weeks if Turks were to get visa-free travel to the 26-nation Schengen area in June.