Greece has scrambled to begin the massive task of implementing a historic EU-Turkey migrant deal aimed at stemming the flow of refugees fleeing war and conflict in the Middle East.

A key part of the agreement, which was sealed on Friday (local time), will take effect from midnight on Sunday when all migrants arriving on the Greek islands will be designated for return to Turkey, a Greek government source told AFP.

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Ahead of the deadline, authorities reported around 1,500 people have crossed the Aegean Sea to Greece’s islands in the past 24 hours, more than double the day before and compared with just several hundred per day earlier in the week.

Hundreds of security and legal experts are set to arrive in Greece to help with implementing the deal, described as “Herculean” by the head of the EU’s executive arm.

With over 40,000 migrants already in Greece, the debt-hit country could not take on this new task without major assistance from its European partners — including the immediate deployment of 2,300 experts — Greek Prime Minister Alexis Tsipras said.

“Four hundred experts in asylum, 400 interpreters and translators and 1,500 security specialists,” Mr Tsipras said, detailing the assistance to be sent to manage the migrant deal approved at an EU summit on Friday.

In practice, the actual return of migrants to Turkey will begin from April 4, according to German Chancellor Angela Merkel — a key backer of the scheme.

For every Syrian refugee expelled from Greece, the deal calls for the EU to resettle one refugee directly from Turkey.

In total some 4,000 border officials and other experts will be needed to carry out the agreement that will cost the EU up to $444 million over six months.

The assistance includes experts who can address the concerns of rights groups who fear the scheme could fail to protect the rights of those refugees, mainly from Syria and Iraq, to seek asylum.

But EU officials have stressed that each application for asylum will be treated individually, with full rights of appeal and proper oversight.

Mr Tsipras also insisted that human rights would be respected.

“We will not make any concessions” in that area, he said.

The United States called the agreement brokered by EU and Turkish officials “an important step” and was confident it “will be implemented in full accordance with EU and international law.”

Around 4,000 people, including many children, have drowned crossing the Aegean Sea from Turkey to Greece in flimsy smugglers’ boats, including 400 from this year alone.