There is growing concern over a lack of preparation for the plan to send migrants back from Greece to Turkey, two days before it goes into effect, BBC reported.
A BBC correspondent who has been to one of the registration sites in Turkey says there are few signs it is ready.
The UN and rights groups have expressed fears about migrant welfare and whether some may be forced back into Syria.
The EU-Turkey deal is aimed at easing the uncontrolled mass movement of people into Europe.
Under the deal, migrants arriving illegally in Greece are expected to be sent back to Turkey from April 4 if they do not apply for asylum or if their claim is rejected.
An EU official said it was hoped that several hundred would be returned on Monday.
For each Syrian returned, a Syrian migrant in Turkey will be resettled in the EU, with priority given to those who have not tried to enter the EU illegally.
Germany said on Friday it would take the first migrants.
Interior ministry spokesman Tobias Plate said that most would be families with children, with the numbers in the "double-digit range".
Last year, more than one million migrants and refugees arrived in the EU by boat from Turkey to Greece.
Tens of thousands have been stranded in Greece after northern countries closed their borders.
Greek officials told media that Monday's returns could start from the island of Lesbos and involve migrants whose asylum claims were considered inadmissible, including those from Pakistan.
Migrants will be taken on buses and put on chartered vessels, under a heavy security escort of one police guard for every migrant, the officials said.
But the BBC's Mark Lowen, who has been to a proposed registration camp near the western Turkish town of Dikili, says the field earmarked for the centre remains empty.
He says local officials are suggesting those sent back here would only stay temporarily in Dikili before being sent to other areas of Turkey.
Meanwhile, he says, boats are still attempting to cross - 160 migrants were caught by the Turkish coastguard near Dikili on Friday and many more actually reached Lesbos.