BRUSSELS (Reuters) - The European Union was in such a rush to start deportations of unwanted migrants from Greek islands to Turkey this week that it skirted legal questions weighing on the scheme as it tries to stem an influx into Europe.
But those issues will return to challenge the EU-Turkey plan once officials move beyond the "low-hanging fruit" of sending back illegal migrants and volunteer returnees to the most controversial deportation of asylum-seekers and refugees.
EU officials aim to send a strong enough deterrent signal to would-be migrants and people smugglers to stanch the flow across the Aegean Sea before any legal challenge can be mounted.
U.N. agencies and rights campaigners denounce the scheme and aim to inform all those who reach the Greek islands on how to apply for asylum.
They may gain moral support when Pope Francis, a vocal critic of Europe's record on welcoming those fleeing war and poverty in the Middle East and beyond, visits the Greek island of Lesbos next week.
The first 202 people were sent back to Turkey from Greece on Monday under a March 18 EU accord with Ankara, which European governments praised as a game-changer but rights advocates say may run against the law.
Though the EU-Turkey deal envisages sending back all refugees and migrants arriving on Greek islands, the EU has so far shied away from returning asylum seekers, recognizing that legal conditions it set for the scheme have yet to be fully met.
"The next landmark ... will be the return of migrants whose applications and subsequent appeals have been rejected," said a person familiar with internal EU deliberations.
Given the political imperative to act fast, "it was agreed that the more clear-cut cases should be dealt with first".
Seeking to defuse criticism by the U.N. human rights and refugee agencies, Brussels placed three basic conditions on sending back everyone, including asylum seekers, to Turkey.
They included changes to Greek law, passed last week, allowing asylum seekers to be sent back to Turkey, and Turkish changes to guarantee protection for returning asylum seekers.
Turkey must still adopt a rule renewing temporary protection to Syrians who would be sent back from the EU and grant protection to non-Syrian asylum-seekers before returns of prospective refugees can start. The EU's top migration official pressed for both provisions in Ankara this week.
So far only people who would have been deported anyway, even without the Turkey accord, have been sent back.
"There is this legal uncertainty and we will see what the courts would say eventually," said one diplomat in Brussels who was involved in the EU-Turkey negotiations. "It would be up to the courts to decide. In some ways it can be prone to challenges - any such development would be a blow.
"But the desired effect seems to be taking place - the people and the smugglers are getting the message that this route will no longer work as it used to, so no point risking it."
The numbers of migrants and refugees arriving on Greek islands from Turkey fell to below 800 a day after March 20, when the deal took effect, and to a trickle this week.
That is well below the several thousand seen daily last September and October at the height of the migration crisis that saw more than one million people reaching Europe last year.
For EU leaders, particularly German Chancellor Angela Merkel, facing increasingly uneasy public opinion at home, it may well be a gamble worth taking.
Any court challenge would be complex and take time. Rights groups say that leaves room for legal shortcomings for the sake of expediency.
Rights advocates question whether the deal really ensures the right to claim asylum and have cases considered individually or respects strict limitations on detaining asylum seekers.
"One main concern is the inescapable conclusion that it's about detaining the people who reach the Greek islands," the U.N. High Commissioner for Human Rights' office said this week.
It cited a "contradiction between the declared goal of returning everyone and the assurances that safeguards and an appeal mechanism will be in place to challenge that", noting that collective expulsions would be illegal.
The new Greek law allows as little as nine days for the whole procedure in which a person who arrives on an island would claim asylum, be rejected, appeal against the decision and get a final ruling. Until then, any return would be suspended.
Pursuing a claim in such a short time seems a tall order given language barriers, migrants' distress, unfamiliarity with EU asylum rules, and organizational chaos on the ground. A lack of interpreters is one potential bottleneck.
"One of the big question marks is whether people understand how to claim asylum. Do they have a real opportunity to make that claim?" said Elizabeth Collett, Director of Migration Policy Institute Europe.
"One thing the EU is going to have to do if it really wants to pursue this and not fall foul of the law is to be over-cautious in ensuring people have access to asylum, legal advice, all the information necessary."
The UNHCR says Greek police have already sent back 13 people who had intended to claim asylum, despite assurances that only irregular migrants had been deported, not potential refugees.
The European Commission insists the Turkey agreement and its implementation are legally sound.
A supporter of the plan, Gerald Knaus of the European Stability Initiative think-tank, said the activists were pursuing their own agendas while the EU-Turkey deal offered a chance for large-scale legal resettlement of refugees in Europe.
"The NGOs and human rights groups... want the deal to fail to discredit it, to nourish the narrative that this is a dirty deal and not in the interest of refugees," he told Reuters.
A rejected asylum-seeker could challenge a return decision in Greek courts, a legal path that could eventually lead all the way to the European Court of Justice or the European Court of Human Rights. But that could take many months or longer.
"In practice ... it may be that access to lawyers and courts is more theoretical than real," said Steve Peers, professor in EU Law and human rights at the University of Essex.
Additional reporting by Michele Kambas in Athens and Paul Taylor in Brussels; Editing by Paul Taylor and Robin Pomeroy