Turkish and EU leaders have gathered in Brussels for an emergency summit on tackling Europe’s worst refugee crisis since World War Two.
The EU has pledged €3bn (£2.3bn; $3.3bn) to Turkey in return for housing migrants and stemming the flow.
Last year, more than a million entered the EU illegally by boat, travelling mainly from Turkey to Greece.
Some 13,000 are stranded on Greece’s border with Macedonia as European states seek to restrict entry.
Nato is expanding its naval mission against people-smuggling in the Aegean Sea to cover Turkish and Greek territorial waters, and will also increase its co-operation with the EU’s border agency Frontex in the region.
The UK has announced that the amphibious landing ship RFA Mounts Bay will join naval vessels from Germany, Canada, Turkey and Greece in the area.
Migrants, many of them fleeing war zones in Syria and Iraq, continue to make the hazardous sea journey from Turkey to Greece’s outer islands.
The human cost of the crisis was brought home again on Sunday when a boat capsized off Turkey with the loss of 25 lives.
EU states are divided over their response to the crisis with strains showing this year even in Germany and Sweden, seen as the countries most open to refugees.
Anti-migrant parties won a general election in Slovakia on Saturday which saw the far right gaining seats.
German Chancellor Angela Merkel and Dutch Prime Minister Mark Rutte met their Turkish counterpart, Ahmet Davutoglu, at the Turkish embassy in Brussels late on Sunday to prepare for the summit.
The 28 EU states are expected to ask Turkey to take back thousands of migrants who do not qualify for asylum.
Last week, European Council President Donald Tusk said he had been told by Turkish President Recep Tayyip Erdogan that his country was ready to take back all migrants apprehended in Turkish waters.
A draft summit statement seen by the Associated Press news agency pledges to “stand by Greece in this difficult moment and will do its utmost to help manage the situation”.
“This is a collective EU responsibility requiring fast and efficient mobilisation,” it adds.
More than 2,000 migrants continue to arrive daily in Greece from Turkey, hoping to reach the richer EU states to the north.
But Macedonia, which aspires to EU membership, is blocking them on its border, now fenced off with razor wire and watchtowers.
A ramshackle tent camp that has grown up around the Idomeni frontier crossing has become the focus of the crisis.
On Sunday, reports from the area said Macedonia had stopped allowing entry to anyone from areas in Iraq and Syria it did not consider to be active conflict zones.
Many migrants in the camp rely on food distributed by volunteers and items like firewood are scarce.
“We have been here five days, or six – who remembers the days anymore?” asked Narjes al Shalaby, 27, from the Syrian capital Damascus, in conversation with AP.
She is travelling with her mother and two daughters, Maria, five, and Bara’a, 10. Her husband and third daughter are already in Germany.
“All we do here is sleep, wake up, sleep,” she said. “We get hungry, we wait in the queue for two hours for a sandwich, we come back, we sleep some more.”
A note on terminology: The BBC uses the term migrant to refer to all people on the move who have yet to complete the legal process of claiming asylum. This group includes people fleeing war-torn countries such as Syria, who are likely to be granted refugee status, as well as people who are seeking jobs and better lives, who governments are likely to rule are economic migrants.