EU leaders are to meet to finalise a deal with Turkey to help ease the migrant crisis.
European Council President Donald Tusk admits a "catalogue of issues" in the way of an agreement remains unresolved.
Turkish Prime Minister Ahmet Davutoglu is due to join the summit on Friday.
The proposed plan would see all migrants arriving in Greece from Turkey sent back. For each Syrian returned, a Syrian in Turkey would be resettled in the EU.
Hundreds of thousands of migrants have travelled through Macedonia over the past year, heading north, the vast majority of them arriving after making the dangerous sea crossing from Turkey.
No 'cast-iron' incentives
At a meeting last week where the plan was initially proposed, EU leaders said that in return for action on the migrant crisis, visa-free travel to Europe for Turkish citizens would be available from June.
They also promised to speed up financial aid promised last year and to make a fresh push on talks over Turkey's eventual membership of the EU.
However, in the latest draft of the deal the incentives being offered to the Turkish government appear to be far less cast-iron than Ankara would like, the BBC's Chris Morris reports from Brussels.
There is no guarantee that Turkey's EU membership bid will be speeded up - only a commitment to prepare for further negotiations as soon as possible.
Secondly, visa-free travel to the Schengen area for Turkish citizens by the end of June will depend on Turkey complying with all 72 conditions that the EU has set out.
Finally, there's no certainty that financial aid for Syrian refugees in Turkey will be doubled to €6bn - only a promise to consider it, if the first tranche of money produces positive results.
In addition to these points of potential disagreement, there is continuing criticism of the legal basis for the idea of returning all irregular migrants from Greece to Turkey, our correspondent adds.
The Spanish foreign minister has said Spain will oppose the "blanket return" of refugees to Turkey.
Despite the obstacles, the head of the European Commission Jean-Claude Juncker said ahead of the summit that he was "cautiously optimistic" a deal could be reached.
Meanwhile UK Prime Minister David Cameron is expected to warn the summit of a possibility of a fresh wave of migrants coming to Europe from Libya this summer.
Although recent attention has been focused on migrants arriving in Greece, the sea route to Italy from Libya has also been a key route for migrants including refugees fleeing conflicts and oppression in Syria, Eritrea and other countries.
Libya has suffered from chronic instability and lawlessness since forces backed by Nato countries including the UK and France overthrew long-serving ruler Col Muammar Gaddafi in October 2011.
Last week, US President Barack Obama suggested Cameron had become "distracted" in the aftermath of the intervention. "I had more faith in the Europeans, given Libya's proximity, being invested in the follow-up," he said.