The migrant agreement between Turkey and the EU officially went into effect on Sunday.
And Greece's coast guard says no migrants crossed the Aegean Sea between Turkey and the Greek islands for the 24 hours ending at 2 p.m. EST Thursday.
Joanna Kakissis, reporting for NPR from Athens, says the sudden drop in numbers was probably connected to the sea conditions: "Authorities said the lack of asylum-seekers arriving to Greece may have more to do with weather ... the Aegean has been hit with gale-force winds since Wednesday."
But there had been a noticeable drop-off in the numbers of migrants crossing even before the bad weather struck.
Last year, about 850,000 people made the crossing, sometimes many thousands in a single day. The Greek authorities say about 2,500 asylum-seekers have arrived on the Greek islands since Monday.
Kakissis says migrants who arrive after Sunday's deadline face deportation to Turkey under the deal with the EU.
There are still some 12,000 people living in appalling conditions at a makeshift camp on the border between Greece and Macedonia. They had hoped to make their way north along the so-called Balkan Trail, but were stranded when Macedonia and other Balkan states closed their borders to migrants.
The Greek government wants to move the asylum-seekers, but they are refusing to go, fearing they will be either stranded in Greece or deported to Turkey.
Meanwhile, there's growing criticism of the EU's migrant agreement with Turkey. Reuters quoted a spokeswoman of the U.N.'s refugee agency, the UNHCR, as saying it will no longer help to transport refugees to processing centers.
"Under the new provisions, these so-called hotspots have now become detention centers," said the UNHCR's Melissa Fleming.
Reuters reports that a number of aid agencies, including the International Rescue Committee and Doctors Without Borders, said they would also withdraw their cooperation.
The European Union's migrant plan has also been rejected by some EU member states. The Polish government says it will no longer accept the nearly 7,000 migrants it had promised to accept. Polish Prime Minister Beata Szydlo cited the attacks in Brussels as the reason for rejecting the plan — but other EU leaders said conflating security concerns with the migrant issue would be playing into the hands of the self-declared Islamic State.