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 aims to stem the flow of migrants and will discuss closing the route north through the Balkans.
It will press Turkey to take back economic migrants and has pledged to give Ankara €3bn (£2.3bn; $3.3bn).
Last year, more than a million people entered the EU illegally by boat, mainly going from Turkey to Greece.
Many migrants leave Greece in a bid to reach northern Europe, but eight countries have introduced temporary border controls.
Some 13,000 migrants are currently stranded in northern Greece, after Macedonia, backed by Croatia, Hungary and Slovenia, closed its border to all but a trickle of migrants.
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The human cost of the migrant crisis was brought home again on Sunday when a boat capsized off Turkey with the loss of 25 lives.
EU states remain 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.
'Stand by Greece'
The summit will be in two parts – the first session from 12:00 GMT will involve Turkey, while in the afternoon UK Prime Minister David Cameron will join other EU leaders in seeking to reach a common approach to the crisis.
Arriving for the summit, German Chancellor Angela Merkel said she anticipated a "difficult discussion".
"This will be one of the more important summits," she said.
A draft summit communique reported by journalists on Sunday had declared that "irregular flows of migrants along the Western Balkans are coming to an end; this route is closed".
However, German government spokeswoman Christiane Wirtz on Monday described this as "speculation", adding: "The negotiations and talks are ongoing and we have to wait."
A German government source told the BBC there was no point saying the route was closed when 300-500 people still arrived in Germany everyday this way.
The EU is expected to ask Turkey to take back thousands of migrants who do not qualify for asylum. In return the EU will discuss plans to resettle in Europe some refugees already in Turkey.
A spokesman for Turkish Prime Minister Ahmet Davutoglu said on Monday that Turkey would do all it could, but added: "It is not possible for any country to stop all migrants to zero."
EU leaders are also likely to raise the issue of the Zaman newspaper. On Friday, a Turkish court ordered the seizure of the opposition journal, increasing fears for media freedom. Two days later it was publishing pro-government articles.
The draft communique also pledges that the EU will "stand by Greece in this difficult moment and will do its utmost to help manage the situation".
Greece said on Monday it would meet its pledge on accommodation for refugees, with a capacity of 37,400 by 15 March.
The EU said last October it would relocate 160,000 asylum seekers, mainly from Greece and Italy, but there was strong opposition among some members and fewer than 700 migrants have moved.
The EU may now overhaul its Dublin Regulation, which requires asylum seekers to lodge claims in their EU country of arrival, and instead adopt a centralised system for processing applications.
The future of the Schengen agreement – which allows passport-free travel in a 26-nation zone – will also be on the agenda. Eight members have introduced temporary border controls, and EU leaders will be anxious to save a system thought to bring billions of euros to Europe's economy every year.
Greek MEP Stelios Kouloglou told the BBC it should not be hard for a continent of 500 million people to absorb one million migrants, but said that in Europe "there is no solidarity. There is no respect of international laws and values".
'All we do is sleep'
More than 2,000 migrants, mostly from Syria, Iraq and Afghanistan, continue to arrive daily in Greece from Turkey.
Macedonia, which aspires to EU membership, is blocking them on its border, now fenced off with razor wire and watchtowers.
The Idomeni frontier crossing, with its ramshackle tent camp, has become the latest focus of the migrant crisis.
One camp resident, Narjes al Shalaby, from the Syrian capital Damascus, told Associated Press: "We have been here five days, or six – who remembers the days anymore?"
She is travelling with her mother and two daughters. 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."
Separately, Nato says it 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.
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.