Mizgin Rashid, 23, is a Syrian refugee from Aleppo. Rashid, who is five months pregnant, poses for a picture outside her shelter at the Ritsona refugee camp in Greece on Jan. 11. (Muhammed Muheisen/AP)

With a cold snap bringing snow and freezing temperatures to Greece’s overcrowded refugee camps, a new type of migrant crisis is overwhelming tens of thousands of people who fled war and poverty in the hopes of a better life in Europe.

The chilly weather this month has already cost the lives of several asylum seekers in the Balkans, as tents and other lightweight shelter that are adequate to Greek islands’ balmy summers have proven inadequate for winter gusts. Heavy snowfall on the islands has piled up on tents, and freezing temperatures have been recorded even on islands that usually have temperate winter weather.

The poor conditions in Greece have highlighted Europe’s ongoing challenge to address the migration crisis, even during winter months in which fresh arrivals have slowed to a trickle because of a forbidding sea crossing. Although the camps have drawn condemnation from the United Nations and senior E.U. leaders, the European Union has left the cash-strapped Greek government to handle the challenge mostly on its own.

“With so many children and vulnerable people remaining in filthy camping tents, the need is great for Europe to show solidarity and take responsibility,” said Roland Schönbauer, an Athens-based spokesman for the U.N. refugee agency. He added that refugees have been wandering through overcrowded camps to keep warm.

Migrant Petram Mehdi, 2, from Tehran, stands by the window of his family’s shelter at the Ritsona refugee camp on Jan. 12. The European Commission says conditions for the refugees are “untenable” during the winter. (Muhammed Muheisen/AP)

At the Pikpa refugee camp on the Greek island of Lesbos, temperatures in recent days have dropped to the low twenties.

“The camp is better than anything we’ve seen before,” said Najwa Hassan, who escaped the Islamic State takeover of Mosul and has been living in the camp since July. “But it’s difficult to keep them warm, they can’t move,” she said, referring to her children.

Hassan said two of her three children lost their ability to walk after the Islamic State threw them off the roof in Mosul. Her husband was beheaded in front of their children; Islamic State militants subsequently beat her up.

Her 15-year-old son, Ahmed, sleeps from dawn to dusk, covered under too few blankets to keep him warm. The scarce moments he’s awake are filled with screaming, Hassan said.

“I don’t know if he’s in pain, or if he’s afraid or maybe only cold,” she said. She said they have not had access to a doctor. At least 15 percent of the population in the refugee camps faces a disability or trauma, according to an estimate from Human Rights Watch, an advocacy organization.

With all of the camps heavily overcrowded, often reaching three times their capacity, aid workers find themselves trying to help with steeply curtailed resources. Some refugees have been forced to take matters into their own hands, burning anything they can find to heat their tents, according to Loic Jaeger, the head of the Greek mission of Doctors Without Borders, which works in the refugee camps.

“We’ve been donating winter clothes, socks and blankets, but what we really need is appropriate shelter, which is something only the authorities can decide on,” Jaeger said.

An elderly Syrian refu­gee walks to his shelter on frozen ground at the Ritsona refugee camp. (Muhammed Muheisen/AP)

In recent days, Greek authorities have offered a temporary solution by converting a tank landing ship into a dorm for some male asylum seekers.

European officials have also condemned the conditions and implored other E.U. nations to step up their aid efforts.

“We all — Greeks, Europeans — have a humanitarian imperative to alleviate the situation here on the islands,” said Dimitris Avramopoulos, the European commissioner charged with migration issues, during a recent visit to Lesbos.

But Greece has been told to cope using its own resources, and a system that would send some asylum seekers back to Turkey to ease pressure on the camps has largely stalled. About 50,000 refugees and migrant are in Greece, according to U.N. refugee agency figures.

The paralysis has frustrated refugee advocates.

“The situation today is the result of eight months not doing enough,” said Jaeger, of Doctors Without Borders. “We all knew that winter would come.”

Michael Birnbaum contributed to this article.