Human Rights Watch (HRW) on Thursday accused Greece and the European Union of arbitrarily detaining and holding under miserable conditions all migrants and potential asylum seekers arriving on the Aegean islands from Turkey.
According to HRW, about 4,000 people, many of them vulnerable - children, pregnant women and people with disabilities - are locked up on the islands of Lesbos and Chios.
The Aegean islands have been the main gateway for close to 1 million migrants, including many refugees from Syria and Iraq, who moved toward wealthy EU countries through Greece and the Balkans in the latter half of 2015.
"The EU's policy, carried out in Greece, has locked up families and others who have fled horrors such as [Islamic State] terror, Taliban threats or Syrian-government barrel bombs," HRW's Eva Cosse said.
She added that there are alternatives to detaining people, so "there is no legal or moral justification to hold asylum seekers and migrants behind bars."
In Brussels, a European Commission spokeswoman, Tove Ernst, told reporters that "detention of asylum seekers and migrants is indeed a possibility, but it should of course only be used very restrictively."
"But it is indeed a possibility which is provided for under EU legislation," she said. "It is a possibility ... which is used on the islands indeed during the procedures in order to make sure that migrants for example do not abscond."
She however highlighted that "returns of migrants and the detention of the migrants ... will be carried out in full respect of EU and international law" and that the EU was working with Greece to make sure all rights of the migrants are respected.
The camps on Lesbos and Chios did start out as open reception and registration centres, but were "converted suddenly to prison-like camps on March 20" following a "deeply flawed EU-Turkey agreement, HRW said.
People who arrived since were detained by Greek authorities, with assistance from the EU border-protection agency Frontex, it added.
Camp Moria on Lesbos, with a population of 3,100, has a perimeter marked off by a triple fence topped off by razor wire. Likewise, a fence and razor wire keep some 1,000 people inside a former aluminum factory on Chios confined to its grounds.
Before they were converted, both camps were serviced by humanitarian organizations and the UN refugee agency UNHCR, but most, including the UN, suspended their work in line with their policy against work in confined facilities.
Conditions in the two camps have since deteriorated, due to overcrowding and lack of service, HRW said.
Ernst also said in Brussels that the European Commission expects Greek authorities to soon start issuing decisions on the asylum requests that have been submitted by Syrians since the EU-Turkey deal came into effect.
So far, Greece has only been returning migrants to Turkey who have not requested asylum. The EU has pledged to assess asylum requests before making a decision on returns to Turkey, but was also waiting for Ankara to implement prerequisite legal changes.
Turkey on Tuesday provided the "necessary additional clarifications," Ernst said.
"It is now expected that the Greek Asylum Service will start issuing admissibility decisions for the Syrians who have been interviewed so far," she added.