The EU and Turkey have ended a summit in Brussels and delayed a decision on how to ease the migration crisis.
Leaders have been discussing an offer by Turkey to do more to stop illegal migration in return for more EU aid and accelerated EU membership talks for Turkey.
They said they had agreed on the broad principles of a plan but needed more time to work on the details of a deal.
Talks will continue ahead of a long-scheduled EU meeting on 17-18 March.
Europe is facing its biggest refugee crisis since World War Two, with most migrants coming via Turkey.
Turkey is currently sheltering more than 2.7 million refugees from the civil war in neighbouring Syria. The EU wants Turkey to take back migrants who do not qualify for asylum and do more to patrol its own waters.
For every migrant sent back, Turkey wants the EU to accept one Syrian refugee from Turkish soil through a resettlement scheme.
The Turkish government is asking for a doubling of EU aid aimed at helping Turkey deal with the crisis, to €6bn ($6.6bn; £4.64bn) as well as a faster path towards EU membership and the speeding up of plans to allow Turks visa-free travel in Europe.
Speaking at a news conference PM Ahmet Davutoglu insisted the extra funds were needed to help refugees in Turkey, and called on countries to share the burden created by the conflict in Syria.
Meanwhile German Chancellor Angela Merkel said the proposals could be a "breakthrough" if realised, stressing that "irregular migration" needed to be turned into "regular migration".
French President Francois Hollande also hailed progress at the talks, indicating that aid to Turkey could be increased.
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After the summit concluded, Luxembourg Prime Minister Xavier Bettel tweeted that summit chairman Donald Tusk would "take forward the proposals and work out the details with the Turkish side before [the 17-18] March" migration summit.
Earlier a spokesman for Hungarian PM Viktor Orban – who has taken a strongly anti-immigrant stance – said he had vetoed the plan to resettle refugees in Europe.
UK PM David Cameron said EU leaders did have "the basis for a breakthrough", which would mean that all migrants arriving in Greece could be returned to Turkey.
However before the summit, he stressed the UK would not take part in any resettlement scheme, saying: "We have an absolutely rock-solid opt-out from these things."
Earlier Turkey's President Recep Tayyip Erdogan accused the EU of failing to deliver on the aid it has already promised.
"It's been four months," Mr Erdogan said in Ankara. "My prime minister is currently in Brussels. I hope he will return with the money."
Last year, more than a million people entered the EU illegally by boat, mainly going from Turkey to Greece.
Many migrants leave Greece in an attempt to reach northern Europe, but eight countries have introduced temporary border controls.
Some 13,000 migrants are currently stranded in northern Greece, after Macedonia closed its border to all but a trickle.
The future of the Schengen agreement – which allows passport-free travel in a 26-nation zone – is on the agenda, as the leaders are anxious to save a system thought to bring billions of euros to Europe's economy every year.
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 union 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.
More than 2,000 migrants, mostly from Syria, Iraq and Afghanistan, continue to arrive daily in Greece from Turkey.
The Idomeni crossing on the Greece-Macedonia border has become the latest focus of the migrant crisis.
One camp resident, Syrian refugee Narjes al Shalaby, told AP news agency: "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.