An aid agency has accused Greece of creating a "humanitarian crisis" for hundreds of illegal migrants detained on the Mediterranean island of Lesbos.
Medecins Sans Frontieres says about 800 migrants are living in overcrowded rooms in the detention centre without proper sanitation or medical care.
Greece has not commented but has previously said it is doing all it can to cope with a huge refugee influx.
It says nearly 10,000 would-be migrants were found on its shores last year.
The total was three times higher than the previous year, the government said.
Last week a separate humanitarian group, Medecins du Monde, accused Greece of neglecting 140 war refugees who had been evicted from their reception centre on the Aegean island of Patmos.
Medecins Sans Frontieres said the migrants in the Lesbos centre were mostly from Africa and Afghanistan.
Yiorgos Karayiannis, head of the organisation's migrant assistance programme for the country, told Reuters news agency the migrants were only allowed out for half an hour every two days and had to endure rooms clogged with stagnant water.
Mr Karayiannis said: "The situation is horrible from a medical point of view. This is an urgent humanitarian crisis."
He said MSF had been working at the centre for two months providing medical assistance and had seen a rise in migrant numbers from about 150 in June to about 800 now.
Last week, Medecins du Monde said 140 war refugees from the Palestinian territories, Afghanistan and Somalia were living rough on the streets of Patmos after being evicted.
They had been living in a cramped discotheque, but the government had not kept up with the rent, the charity said.
Greece has only six detention centres for illegal migrants and says it does not have sufficient resources to handle the influx.
Athens says illegal immigration is a pan-European problem and accuses Greece's EU partners of not doing enough to support it on the front line.