Smuggled migrants to leave Greece from Sunday onward

The agreement is "aimed at stopping the flow of irregular migration via Turkey to Europe," Tusk said.

The measure will take place in two steps. Starting Sunday, arriving migrants will be screened and the ones deemed irregular will have to be returned.

But the first returns will only take place when Greece is ready to examine all asylum demands and organise the transport to Turkey.

Over 800,000 people arrived on the Greek islands last year from Turkey, sparking political rifts within the EU as others imposed internal border checks within the Schengen passport free zone.

According to the plan, for every one Syrian returned to Turkey, the EU will resettle from Turkey a Syrian refugee.

The mechanism will start when Greece is ready to return the first migrants. On the same day the first refugees will be resettled to Europe.

'Self-destruction clause'

As some EU countries feared that this so-called one-to-one scheme would be open-ended, the resettlement cap has been provisionally placed at 72,000.

What one EU official called a "self-destruction clause" was also introduced.

"Should these arrangements not meet the objective of ending the irregular migration and the number of returns come close to the numbers provided for above, this mechanism will be reviewed," the EU-Turkey statement outlining the plan says.

No date has been set to decide whether the scheme, which will be assessed every month, works or not and when it could be ditched.

Both sides vow there will be no collective explusion. Anyone who doesn't apply for asylum or whose application is unfounded will be sent packing.

'Herculean task'

But outstanding questions remain on how Greece intends to process the asylum claims and appeals in record time while at the same time respecting EU and international laws and standards.

In an indication of the importance of the deal, the European Commission said they would help mobilise some 4,000 people to assist Greece in the endeavour.

"We need case workers, interpreters, judges, return officers and security officers," said commission president Jean-Claude Juncker.

Maarten Verwey, a senior EU commission official, has been appointed as EU coordinator in Athens. He will be tasked to make sure the plan becomes operational.

Every person who arrives in Greece will still have to be assessed on an individual basis and will also have the right to an appeal.

Cost estimates over a six-month period hover around €300 million.

"Greece is faced by a Herculean task," admitted Juncker.

"Many countries are sceptical but it is in Greece's self-interest to deliver," the other EU official said.

Once flows from Turkey to the Greek islands stopped or slowed, the EU will trigger a "voluntary humanitarian admission scheme".

Plans are also under way for the EU states to start relocating some 6,000 asylum seekers per month.

The move is part of a stalled EU relocation scheme launched last September that has so far failed to deliver.

Pressed on why EU states would start relocation, given the poor trackrecord, Juncker said he had received assurances among member state heads at the summit.

Meanwhile, Turkey is walking away from the deal with less than it had initially demanded.

'Irreversibility'

But PM Davutoglu remained optimistic, describing the deal as "historic day" between Turkey and the EU.

Turkey wanted to open up five EU accession negotiation chapters but managed to get away with just one, chapter 33 (on budgetary affairs) before the end of June, following strong opposition from Cyprus.

Meanwhile, Ankara managed to secure some of concessions and hopes to see short-term visa restrictions waived for Turkish nationals on visit in EU states that belong to the Schengen area.

Turkey first has to meet 72 requirements from the EU for the waivers to be lifted. It has so far met 37.

"We hope it will be before the end of June," said Davutoglu.

The EU-Turkey agreement comes after 10 days of intense negotiations following a summit on 7 March when the plan was surprisingly put on the table by Turkey with the support of Germany and the Netherlands.

Irreversibility

"We have found an agreement that has a certain component of irreversibility," German chancellor Angela Merkel said.

She said that now Europe would be "able to manage this very difficult test" that the refugee crisis is. But she admitted that difficulties still stayed ahead.

"I'm under no illusion whatsoever that what we agreed will not meet with setbacks, enormous logistical challenges that we have to contend with," she said.