The numbers of migrants landing in Greece from Turkey is starting to creep up again, showing efforts to close off the route are coming under strain, the International Organization for Migration (IOM) said on Friday.
Around 150 people a day had arrived over the last three days, still way off the numbers seen a month ago, the organization added, but showing an increase since an EU deal with Turkey deal to stem the flow.
"The arrivals in Greece which were down to literally zero some days this month, are beginning to creep back up," IOM spokesman Joel Millman told a Geneva news briefing.
"It could be the weather, it could be any number of things, it could be that smugglers are getting more creative."
Europe signed an agreement with Turkey last month to close off the main route into Europe for more than a million people, most fleeing war and poverty in the Middle East, Asia and Africa.
NATO sent ships into Greek and Turkish waters in the Aegean in March, though Greek Prime Minister Alexis Tsipras said on Friday that Turkish demands were hampering the mission.
"It could be that there is just still a lot of demand in Turkey ... people have already spent months to get to Turkey and where there is a will and where there is means, people will try to satisfy them," Millman told the briefing.
"It still shows that hermetic sealing that seemed to be happening a month ago isn't anymore."
There were also signs of increased numbers of people from sub-Saharan Africa taking the perilous route across the Mediterranean to Europe, he said.
More than 3 million people have been displaced in the Lake Chad basin - in Nigeria, Niger, Cameroon and Chad - by violence by the militant group Boko Haram, he added.
An estimated 180,245 migrants and refugees have entered Europe by sea so far this year, the bulk of 154,227 to Greece, while at least 1,232 have died, the IOM said.
Victims include an estimated 500 people who drowned in a shipwreck off Libya this week, mainly from the Horn of Africa, which 41 people survived.
For the first three months of the year, an average of nearly 1,500 migrants or refugees arrived by sea each day in Europe, according to the agency.