The pressure to create a binding agreement was high going into the two-day summit in Brussels after repeated failed attempts to forge a way forward.
European Council President
"I am cautiously optimistic, but frankly more cautious than optimistic," he said in a statement Thursday morning, hours before the leaders were due to arrive for the talks.
A stream of refugees has arrived on European shores in the last year, the majority fleeing their homes in war-torn Syria, Iraq, Afghanistan and other countries in the Middle East and North Africa.
Turkey has become a key route into Europe, and more than 1 million people have crossed from there by boat into Greece since January 2015, according to United Nations figures.
More than 143,000 have made that journey this year alone.
Unable to control the
A proposed deal centers around Turkey working to prevent migrants from leaving its shores and agreeing to take back any new arrivals who land in Greece. For every person sent back, EU countries could relocate one Syrian refugee from Turkey, up to a total of 70,000.
As he arrived at the summit, British Prime Minister David Cameron said he was committed to "busting the people smugglers" and "breaking the link between getting in a boat and getting settlement in Europe."
However, the so-called "one in, one out" system has been widely renounced by human rights groups and United Nations agencies that say it violates the fundamental principle of asylum and may be illegal.
In return for Turkey's cooperation, the country's prime minister, Ahmet Davutoglu, asked for the resumption of its long-stalled EU membership talks and the easing of EU visa restrictions for Turkish citizens.
Turkey has also asked for a doubling in aid — from 3 billion euros to 6 billion euros (more than $6.5 billion) — to enable Turkey to assist the 2.7 million Syrian refugees currently living there.
This week's summit is the latest attempt by the 28-member EU bloc to deal with the flow of men, women and children. Thousands of migrants have died making the journey.
The crisis has exposed deep divisions between European countries with conflicting interests, financial pressures and sensibilities towards migrants.
Some countries along the Balkan route have taken to imposing their own ad hoc border controls.
As a result, an estimated 40,000 migrants have been stranded in Greece after passage north through Macedonia was blocked.
Plans to resettle 160,000 refugees throughout EU nations has also largely failed, with only 937 actually being relocated from Greece and Italy.
European Parliament President Martin Schulz said he thought it was "relatively hypocritical" that some member states were critical of the European Commission's negotiations with Turkey.
"There is a lot to do, and I hope that we can agree today with Turkey on the one hand and also the member states among themselves," Schulz told reporters as he arrived at the summit.
EU leaders started to arrive in Brussels in the late afternoon. The migrant crisis will be debated over a working dinner that is expected to run late into the evening.
Davutoglu has been invited to meet the EU leaders Friday to try to formalize an agreement between the European Union and Turkey.
In an open letter to members of the European Council, Tusk warned that "the catalogue of issues to be resolved before we can conclude an agreement is long."
He also said that any deal struck must meet international and European law, and be agreeable to all member states.
That final point could prove difficult, as Cyprus has threatened to block any agreement over a long-standing feud with Turkey, which refuses to recognize the island state.