EU leaders on Thursday kicked off difficult talks on a contentious plan to stem migration flows from Turkey to Europe, with several warning that a deal was uncertain.

The plan is the bloc's latest effort to end a surge of migrants and asylum seekers that saw more than 1 million people reach European shores last year. Most crossed the Aegean Sea from Turkey to Greece, aiming to then move on to wealthy northern states.

The European Union hopes that its leaders can strike a new migration deal with Turkey by Friday that will end the Aegean flows. But the agreement is proving legally challenging and has been lambasted by rights activists, while also meeting with resistance from several EU capitals.

"I understand and also support part of the criticism because I think that the proposed package is very complicated, will be very difficult to implement and it is on the edge of international law," Lithuanian President Dalia Grybauskaite said as she arrived in Brussels for the start of a two-day EU summit.

Belgian Prime Minister Charles Michel warned that he would go for "no agreement at all rather than a bad agreement."

EU President Donald Tusk said he was "cautiously optimistic - but frankly speaking more cautious than optimistic" of finalizing the plan with Turkey this week. German Chancellor Angela Merkel said she agreed with this assessment.

"We do understand this is quite a Herculean task we have, but we need to make it work," Estonian Prime Minister Taavi Roivas added. "We're here to seal the deal and we will work all night if needed."

The new deal centres on Turkey helping to stem the Aegean flows in exchange for closer ties with the EU.

Much of the criticism from rights activists has focused on a planned migrant swap that would see the EU send Syrian asylum seekers from Greek islands back to Turkey, as part of a bid to undercut migrant smuggling networks.

Dutch Prime Minister Mark Rutte predicted that this would lead the Aegean flows to "stop in three to four weeks." British Prime Minister David Cameron called the planned approach "a good idea."

Europe's leading rights watchdog, the Council of Europe, endorsed the approach as long as asylum applications for those arriving on Greek islands are fully processed before a decision is made on whether to send them back to Turkey.

Others remain sceptical. German refugee aid organization Pro Asyl said it is a "farce" to expect Greece to adequately process applications when it has a "de facto non-existent asylum system."

Thousands of people took to the streets of Spain on Wednesday, calling the planned deal a "disgrace." All the political parties in the Spanish parliament have agreed on the need to reject the pact with Turkey.

"The refugees are fleeing from terror and war and cannot here come upon a wall of indifference, a wall of European denial," Socialist leader Pedro Sanchez said this week.

Cyprus is considered to be another potential stumbling block. The island, which is in the midst of delicate reunification talks with its Turkish-backed north, has been resisting Ankara's demands for quick progress on its long-running EU membership bid.

Irish Prime Minister Enda Kenny said it is "unlikely that the accession chapters will be expedited."

Cypriot President Nicos Anastasiades is insisting that Turkey first implement agreements that would for instance allow Cypriot vessels to enter Turkish ports, despite the long-standing tensions between the two sides.

"It's so easy – if Turkey will fulfill its obligations, ... then there is no problem. But without it, we could do nothing," Anastasiades said as he arrived for the summit.

A Turkish quest to have its citizens get visa-free access to Europe by June is also proving sensitive. The EU is insisting that Ankara first meet prerequisite conditions.

"It is clear that we won't start haggling about the conditions of visa liberalization," Rutte said. "They want to speed it up? Fine, but they have to show they can speed up too to meet the 72 benchmarks."

"I don't accept negotiations that sometimes resemble a form of blackmail," Michel added. "We will not be duped."

The 28 EU leaders will meet with Turkish Prime Minister Ahmet Davutoglu on Friday.

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