A call has been made to help thousands of refugees and other migrants in camps on Greece's eastern Aegean island of Lesbos, who face harsh winter conditions.
Dimitris Avramopoulos, the European Union's commissioner for migration, said solutions must be found immediately.
"We all... have a humanitarian imperative to alleviate the situation here on the islands," Mr Avramopoulos said during a visit to Lesbos, accompanied by Greece's migration minister.
"It simply cannot be that refugees are left out in the cold, to brave the worst of winter without a roof over their heads."
More than 5,000 people live in overcrowded camps on Lesbos, and hundreds had to be evacuated from tents covered by snow during a cold spell this month. Many were transferred to a Greek naval vessel.
Resentment is rising among many Lesbos residents, who have faced the brunt of Europe's immigration crisis for the past two years and fear their tourism-reliant island will remain an open prison for migrants indefinitely.
More than 62,000 refugees and other migrants have been stuck in Greece since a series of Balkan border closures and an EU-Turkey deal on stemming migration.
These include about 15,000 who reached the eastern islands after the March 2016 agreement, and are not allowed to travel to the mainland unless they successfully apply for asylum. If they cannot prove that they merit asylum in Greece, rather than neighbouring Turkey from which they travelled, they face being returned to Turkey.
Mr Avramopoulos promised to reduce congestion in island camps by moving vulnerable people to the mainland and stepping up returns to Turkey.
Also on Wednesday, the Human Rights Watch organisation claimed that Greek authorities, as well as the United Nations refugee agency and international charities receiving significant funding to operate Greece's camps, are failing disabled migrants.
"Asylum-seekers and other migrants with disabilities have particular difficulties getting basic services such as shelter, sanitation, and medical care, and... have limited access to mental health care," the group said.
"For example, one older woman who uses a wheelchair had not been able to take a shower for a month."
Meanwhile, a Serbian government official said about 1,100 migrants remain out in the open in severe weather after only 250 people agreed to move away from the streets into a camp outside Belgrade, the capital.
Svetlana Velimirovic, from the government's refugee agency, said that Serbia's asylum centres have enough room for all migrants but that most have refused to go.
Migrants trapped in Serbia have been reluctant to seek official help, fearing they could be deported or unable to move towards Western Europe.
Hundreds of men have occupied a sprawling abandoned warehouse by the Belgrade bus station, enduring freezing temperatures, snow and cold wind, without any facilities.
Aid groups have criticised the Serbian government for letting the migrants stay out during the winter. About 7,000 people remain stranded in the Balkan country.