A plan to return all irregular migrants arriving on Greek islands back to Turkey could take effect on Sunday if EU leaders strike a deal on Friday with the Turkish prime minister, senior EU officials said.

German Chancellor Angela Merkel warned overnight that any delay in imposing deportation orders on people arriving from Turkey could be a "pull factor", generating a rush to reach Greece before the system is put in place.

Leaders are still discussing exactly when the cutoff might be, officials said, and actual expulsions will take much longer.

Greek officials say they will need weeks to amend legislation and prepare asylum tribunals and other facilities on the islands in order to meet legal requirements of the deal before actually sending migrants and refugees back to Turkey.

Setting a launch date of Sunday would mean people arriving after the cutoff would have to be held on the islands until their cases can be handled. An average of 1,157 people a day have been arriving this month, well down February's mean of nearly 2,000, but Athens is concerned about accommodating large numbers on small islands before mass expulsions could begin.

EU officials concede that if large numbers continue to make the crossing to Greece, the scheme is unlikely to be practical. They see its essence as sending a deterrent signal to refugees not to leave Turkey but to remain and apply via legal routes for up to 72,000 resettlement places the EU is offering initially.

EU leaders will meet Turkish Prime Minister Ahmet Davutoglu on Friday in the hope of agreeing the deal. Difficulties remain over whether Turkey can meet EU legal requirements for refugees to be sent back there and whether offers of financial and political concessions from Europe can persuade Ankara to help.