The head of the charity Doctors Without Borders on Wednesday called for authorities to "urgently evacuate" vulnerable migrants from badly overcrowded camps on Greek islands.
Christos Christou told reporters in Brussels he visited the camps last week and saw desperate conditions on par with what is normally seen in war zones and natural disasters.
Coupled with the dire situation for migrants trying to get to Europe from Libya or in eastern Europe, "thousands are living in inhumane conditions ahead of winter", he said.
The organisation, known by its French initials MSF, is one of several non-governmental bodies sounding the alarm over the situation in the Greek island camps.
Those centres, holding more than 39,000 migrants but built for fewer than 7,000, have become facilities where asylum-seekers -- many from Afghanistan and Syria -- languish for months or years while their claims on refugee status are processed.
The UN refugee chief on Wednesday said Athens must take "urgent" steps to improve living conditions in the camps.
"Whatever the situation of these people, we cannot accept that they live in such miserable conditions, exposed also to violence and exploitation," United Nations High Commissioner for Refugees Filippo Grandi said after visiting camps on Lesbos.
In 2106, the European Union struck a deal with Turkey that cut numbers trying to enter Europe through Greece, but the migrant flow has started to swell again this year.
Turkish President Recep Tayyip Erdogan has threatened to scupper the deal by "opening the gates" to Europe for millions of refugees unless he wins international support for his military incursion into northern Syria.
Greece last week said it plans to overhaul its island camps to turn them into detention centres while asylum-seekers have their files evaluated.
Prime Minister Kyriakos Mitsotakis has also pledged to quickly resettle over 4,000 unaccompanied children
MSF urged Greek authorities to move women, children and the ill from the island camps to the Greek mainland, where another 22,000 people are already sheltering in camps nearly full or past capacity.
"We urge them to evacuate the most vulnerable people from these camps," Christou said.
He also urged other EU countries to take in some of the thousands of migrants in Greece -- something many in the European bloc are reluctant to do.afp