European Union leaders have sought to press Turkey to do more to stop migrants from entering Europe and to shore up support for Greece, where thousands of people are stranded.
The leaders are expected to acknowledge that the main Balkan migrant route is effectively closed, after Macedonia - backed by Austria, Croatia, Slovenia and Hungary - limited border crossings to a trickle.
Hundreds of thousands of people fleeing conflict or poverty have used the route in recent months to try to reach preferred destinations like Germany or the countries of Scandinavia.
Ahead of the summit in Brussels, thousands of people waited by Greece's border with Macedonia hoping to be allowed to cross.
"Whatever it takes. We will go. We have nothing to go back to. Our homes are destroyed," said Lasgeen Hassan, who, with the rest of his Kurdish Syrian family, wants to reunite with relatives in Germany.
Arriving for early talks with Turkish Prime Minister Ahmet Davutoglu, Greece's prime minister urged his EU partners to put long-agreed and long-delayed migrant plans into action.
"Rules are for all, and everybody has to implement our common decisions," Prime Minister Alexis Tsipras told reporters. "If there are agreements that are not implemented there are not agreements at all."
EU leaders agreed in September to share 160,000 refugees arriving in Greece and Italy over two years.
As of March 3, fewer than 700 people had been relocated to other European countries.
In a draft statement prepared for the summit, seen by The Associated Press, the leaders said they will pursue "comprehensive, large scale and fast-track returns to Turkey of all irregular migrants not in need of international protection".
Turkey is home to an estimated 2.75 million refugees, most from Syria, and rights groups say that many are living in tough conditions.
Mr Davutoglu said he hopes the summit will mark a turning point in EU-Turkey ties. He said the meeting is as focused on Turkey's future EU membership as on the refugee emergency.
"Turkey is ready to work with the EU, and Turkey is ready to be a member of the EU as well," Mr Davutoglu told reporters, expressing hope that the talks "will be a success story and a turning point in our relations".
The EU is desperate to halt the flow of migrants crossing the Aegean Sea between Turkey and Greece. It has offered Turkey €3bn in refugee aid, fast-track EU membership and an easing of visa rules to win its support.
Dutch Prime Minister Mark Rutte, whose country holds the EU's rotating presidency, said Turkey will need extra help if more migrants are sent back.
"We will need to bring relief to Turkey, and that means you have to be willing to take people in from Turkey" who will probably qualify for asylum, he said.
Human rights group Amnesty International hit out at the leaders for using Turkey as a buffer to stop migrants, calling the move "a dangerous and deliberate ploy to shirk their responsibilities to people fleeing war and persecution".