Refugees and migrants at two Aegean islands camps are being automatically detained in wretched prison-like conditions, a human rights group said Monday.

New York-based Human Rights Watch (HRW) said the forced detention of about 4,000 people - including women with children, unaccompanied minors and elderly people - in fenced off camps on the Greek islands of Lesbos and Chios "amounts to arbitrary detention."

"The EU's policy, carried out in Greece, has locked up families and others who have fled horrors such as ISIS ("Islamic State") terror, Taliban threats, or Syrian government barrel bombs," said Eva Cossé, Greece specialist at HRW.

"When alternatives to detention exist, as they do on the Greek islands, there is no legal or moral justification to hold asylum seekers and migrants behind bars."

The Aegean Islands have been the gateway to Europe for hundreds of thousands of refugees and migrants fleeing war and poverty in the Middle East and Asia.

Since a migrant deal between the EU and Turkey went into effect on March 20, HRW said Greek authorities turned open reception and registration centers into guarded camps. Many refugees and migrants were not informed why they were being detained, HRW said based on interviews with camp residents.

Under the migration deal Turkey agreed to take back migrants who crossed the Aegean to Greece in exchange for Europe taking Syrian refugees directly, as well as financial aid, visa liberalization and the acceleration Turkey's EU talks. But the migrants are still arriving in Greece at a greater rate than they can be deported, putting a strain on already overcrowded camps.

Nearly 3,100 people held at the Moria facility on Lesbos and another 1,000 people at the VIAL facility on Chios are receiving poor food, unhealthy and insufficient water and have limited access to health care, according to HRW.

Before being converted into closed camps, humanitarian organizations and the UN refugee agency were providing assistance and support, but they pulled out in late-March to protest the EU-Turkey deal. Since then services in the overcrowded camps have deteriorated, HRW said.

The European Commission spokeswoman Tove Ernst told reporters in Brussels on Thursday 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."

Previously, most migrants would avoid registering or applying for asylum in Greece in the hope of getting protection in richer northern European countries. But now that trend is reversing due to a policy that sees those who haven't applied for asylum in Greece being deported to Turkey.

The EU and Greece have pledged to process all asylum applications before making a decision to deport people back to Turkey.