Migrants are reflected in a water puddle as they disembark a Greek coast guard vessel, off the coast of the Greek island of Lesbos, Monday, March 21, 2016. The number of stranded migrants in Greece exceeded 50,000 Monday as the number of daily arrivals showed no sign of dropping. (AP Photo/Petros Giannakouris)
The Associated Press
ATHENS, Greece (AP) - Greece detained hundreds of refugees and migrants on its islands Monday, as officials in Athens and the European Union conceded a much-heralded agreement to send thousands of asylum-seekers back to Turkey is facing delays.
Migrants who arrived after the deal took effect Sunday were being led to previously open refugee camps on the islands of Lesbos and Chios and held in detention, authorities on the islands said.
EU countries are trying to avoid a repeat of the mass migration in 2015, when more than a million people entered the bloc. Most were fleeing civil war in Syria and other conflicts, traveling first to Turkey and then to the nearby Greek islands in dinghies and small boats. Efforts to limit migration have run into multiple legal and practical obstacles.
Under the deal, Greek authorities will detain and return newly arrived refugees to Turkey. The EU will settle more refugees directly from Turkey and speed up financial aid to Ankara. The two sides, however, are still working out how migrants will be sent back.
"We are conscious of the difficulties," EU Commission spokesman Margaritis Schinas said in Brussels.
"And we are working 24-7 to make sure that everything that needs to be in place for this agreement to be implemented soon is happening."
Commission officials said support staff needed to implement the deal - including hundreds of translators and migration officers - would not start arriving until next week. Returns, they said, cannot start until Greece changes its law to recognize Turkey as a "safe country" for asylum applications.
The human rights group Amnesty International sharply criticized the plan.
"Turkey does not offer adequate protection to anyone," Iverna McGowan, the head of Amnesty's EU office, told The Associated Press, accusing Turkey of routinely forcing Syrians back across the border.
Migrants, meanwhile, continued to reach Greek islands in large numbers, as smugglers appeared to be opting for more overnight crossings and increasingly dangerous routes.
Four people died Sunday while trying to reach Greece, two men off the island of Lesbos and two girls off the tiny islet of Ro, the coast guard said. More than 1,600 people made the crossing to Greece on Sunday, and 262 were rescued in five incidents after vessels sank or were in distress.
"We face an uphill effort. Implementation of this agreement will not be an easy issue," Greek Prime Minister Alexis Tsipras said.
He met in Athens with EU Migration Commissioner Dimitris Avramopoulos and urged him to increase pressure on Turkey to crack down on smugglers.
Migrants and refugees have been stranded in Greece since Austria and Balkan countries started border closures in recent weeks. The number stranded reached 50,000 on Monday, with some 12,000 still camped out on Greece's closed border with Macedonia.
On the Greek mainland, army personnel expanded refugee shelters at sites in the central and northern parts of the country - mostly at former army bases - so migrants who traveled to the Greek islands before the agreement came into effect could be resettled.
"We are creating between 500 and 1,000 additional shelter places every day ... and the total right now is 36,000," Migration Minister Ioannis Mouzalas said.
On Monday, monitors from Turkey's interior Ministry arrived on Lesbos, Chios and three other Greek islands to help supervise the agreement and were to stay for at least one week.
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Cook reported from Brussels. Raf Cassert in Brussels, Suzan Fraser in Ankara, Turkey, and Costas Kantouris in Thessaloniki, Greece, contributed to this report.
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Maysa Elmohamed holds her 7 day old baby Basel al Sbeihi, as they rest in the makeshift refugee camp at the northern Greek border point of Idomeni, Greece, Monday, March 21, 2016. Hundreds of mostly Syrian asylum-seekers continued to arrive in Greece by sea Sunday despite the start of an international agreement to send migrants back to Turkey. (AP Photo/Darko Vojinovic)
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People gather at the Athens' port of Piraeus, where over 5,000 refugees and migrants stay, on Monday, March 21, 2016. Greek Prime Minister Alexis Tsipras says an international agreement to limit the number of refugees and migrants traveling to Europe cannot be properly implemented unless smugglers on the Turkish coast are stopped. (AP Photo/Thanassis Stavrakis)
The Associated Press
A migrant boy sits in front of the police cordon during a protest demanding the opening of the border between Greece and Macedonia in the northern Greek border station of Idomeni, Greece, Monday, March 21, 2016. Hundreds of mostly Syrian asylum-seekers continued to arrive in Greece by sea Sunday despite the start of an international agreement to send migrants back to Turkey. (AP Photo/Darko Vojinovic)
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African migrants covered with blankets sit on a bus a short while before they are taken to Moria camp, after they were rescued by the Greek coast guard off the coast of the Greek island of Lesbos, Monday, March 21, 2016. The number of stranded migrants in Greece exceeded 50,000 Monday as the number of daily arrivals showed no sign of dropping. (AP Photo/Petros Giannakouris)
The Associated Press
A man gets a haircut from a fellow migrant at the northern Greek border point of Idomeni, Greece, Sunday, March 20, 2016. German Chancellor Angela Merkel is urging migrants in the squalid tent city at Idomeni, on the Greek-Macedonian border, to trust Greek authorities and leave for better accommodation as thousands are still staying on site after the closure of Macedonia's border. (AP Photo/Vadim Ghirda)
The Associated Press
A migrant washes her hands at a makeshift refugee camp at the northern Greek border point of Idomeni, Greece, Monday, March 21, 2016. Hundreds of mostly Syrian asylum-seekers continued to arrive in Greece by sea Sunday despite the start of an international agreement to send migrants back to Turkey. (AP Photo/Darko Vojinovic)
The Associated Press
A migrant boy looks through a door at a train station near a makeshift refugee camp at the northern Greek border point of Idomeni, Greece, Monday, March 21, 2016. Hundreds of mostly Syrian asylum-seekers continued to arrive in Greece by sea Sunday despite the start of an international agreement to send migrants back to Turkey. (AP Photo/Darko Vojinovic)
The Associated Press
Migrants rest at a train station near a makeshift refugee camp at the northern Greek border point of Idomeni, Greece, Monday, March 21, 2016. Hundreds of mostly Syrian asylum-seekers continued to arrive in Greece by sea Sunday despite the start of an international agreement to send migrants back to Turkey. (AP Photo/Darko Vojinovic)
The Associated Press
A migrant girl plays at a makeshift refugee camp at the northern Greek border point of Idomeni, Greece, Monday, March 21, 2016. Hundreds of mostly Syrian asylum-seekers continued to arrive in Greece by sea Sunday despite the start of an international agreement to send migrants back to Turkey. (AP Photo/Darko Vojinovic)
The Associated Press
A migrant plays with a child at a train station near a makeshift refugee camp at the northern Greek border point of Idomeni, Greece, Monday, March 21, 2016. Hundreds of mostly Syrian asylum-seekers continued to arrive in Greece by sea Sunday despite the start of an international agreement to send migrants back to Turkey. (AP Photo/Darko Vojinovic)
The Associated Press
A migrant child stands under clothes hung to dry at a makeshift refugee camp at the northern Greek border point of Idomeni, Greece, Monday, March 21, 2016. Hundreds of mostly Syrian asylum-seekers continued to arrive in Greece by sea Sunday despite the start of an international agreement to send migrants back to Turkey. (AP Photo/Darko Vojinovic)
The Associated Press
Migrant children play at a train station near a makeshift refugee camp at the northern Greek border point of Idomeni, Greece, Monday, March 21, 2016. Hundreds of mostly Syrian asylum-seekers continued to arrive in Greece by sea Sunday despite the start of an international agreement to send migrants back to Turkey. (AP Photo/Darko Vojinovic)
The Associated Press
A volunteer warms herself by a makeshift fire as she waits for more refugees and migrants to arrive in the northeastern Greek island of Lesbos, Monday, March 21, 2016. The number of stranded migrants in Greece exceeded 50,000 Monday as the number of daily arrivals showed no sign of dropping. (AP Photo/Petros Giannakouris)
The Associated Press
A migrant looks out of a bus window while others are covered with blankets a short while before they are taken to Moria camp, after they were rescued by the Greek coast guard off the coast of the Greek island of Lesbos, Monday, March 21, 2016. The number of stranded migrants in Greece exceeded 50,000 Monday as the number of daily arrivals showed no sign of dropping. (AP Photo/Petros Giannakouris)
The Associated Press
Maysa Elmohamed holds her 7 day old baby Basel al Sbeihi, as they rest in the makeshift refugee camp at the northern Greek border point of Idomeni, Greece, Monday, March 21, 2016. Hundreds of mostly Syrian asylum-seekers continued to arrive in Greece by sea Sunday despite the start of an international agreement to send migrants back to Turkey. (AP Photo/Darko Vojinovic)
The Associated Press
A Greek police officer closes the entrance of the Moria camp a temporary shelter fro migrants and refugees, Monday, March 21, 2016. The number of stranded migrants in Greece exceeded 50,000 Monday as the number of daily arrivals showed no sign of dropping. (AP Photo/Petros Giannakouris)
The Associated Press
A family embark a bus after their arrival by ferry with hundreds of other migrants and refugees from the northeastern Greek island of Lesbos to the port of Elefsina, west of Athens, Monday, March 21, 2016. The number of stranded migrants in Greece exceeded 50,000 Monday as the number of daily arrivals showed no sign of dropping after an agreement aimed at limiting the number of refugees and migrants traveling to the European Union. (AP Photo/Thanassis Stavrakis)
The Associated Press
Migrants wait for food in the make shift refugee camp at the northern Greek border point of Idomeni, Greece, Monday, March 21, 2016. Hundreds of mostly Syrian asylum-seekers continued to arrive in Greece by sea Sunday despite the start of an international agreement to send migrants back to Turkey. (AP Photo/Darko Vojinovic)
The Associated Press
Migrants wait for food in the makeshift refugee camp at the northern Greek border point of Idomeni, Greece, Monday, March 21, 2016. Hundreds of mostly Syrian asylum-seekers continued to arrive in Greece by sea Sunday despite the start of an international agreement to send migrants back to Turkey. (AP Photo/Darko Vojinovic)
The Associated Press
A man sleeps on the ground as children play outside their tents at the Athens' port of Piraeus, where over 5,500 refugees and migrants stay, on Monday, March 21, 2016. Greek Prime Minister Alexis Tsipras says an international agreement to limit the number of refugees and migrants traveling to Europe cannot be properly implemented unless smugglers on the Turkish coast are stopped. (AP Photo/Thanassis Stavrakis)
The Associated Press
A migrant boy shouts slogan during a protest demanding the opening of the border between Greece and Macedonia in the northern Greek border station of Idomeni, Greece, Monday, March 21, 2016. Hundreds of mostly Syrian asylum-seekers continued to arrive in Greece by sea Sunday despite the start of an international agreement to send migrants back to Turkey. (AP Photo/Darko Vojinovic)
The Associated Press
Migrants, right, shout slogans in front of police officeres while blocking a railway during a protest demanding the opening of the border between Greece and Macedonia in the northern Greek border station of Idomeni, Greece, Monday, March 21, 2016. Hundreds of mostly Syrian asylum-seekers continued to arrive in Greece by sea Sunday despite the start of an international agreement to send migrants back to Turkey. (AP Photo/Darko Vojinovic)
The Associated Press
The sun rises as migrants and refugees on a dingy arrive at the shore of the northeastern Greek island of Lesbos, after crossing the Aegean sea from Turkey on Sunday, March 20, 2016. In another incident two Syrian refugees have been found dead on a boat on the first day of the implementation of an agreement between the EU and Turkey on handling the new arrivals. (AP Photo/Petros Giannakouris)
The Associated Press