U.N. agencies on Monday urged authorities in Bosnia-Herzegovina to help migrants and asylum seekers who are stranded in freezing winter temperatures.

The agencies and humanitarian partners said authorities must act quickly, echoing warnings over the weekend from the U.N. International Organization for Migration and Office of the High Commissioner for Refugees.

The Lipa refugee camp in northwest Bosnia-Herzegovina, for example, was closed last week after a fire left hundreds without shelter, and other shelters face severe overcrowding. The Lipa camp was built earlier this year but wasn’t prepared for winter.

“With the recent heavy snowfalls and temperatures below freezing, up to 500 people currently stranded at the location of the former Lipa camp are at immediate safety, health and protection risk,” the agencies said in a joint statement.

“With no heating at the site, frostbite, hypothermia and other severe health problems are already being reported by those stranded at the location.”

They said at least 2,000 others have been forced to take shelter in abandoned buildings and makeshift camps. “Failing to act with the utmost urgency will put lives at risk,” they added.

“Migrants are not allowed to leave site, and are now making fires inside the remaining tents to keep warm,” IOM Chief of Mission Peter Van der Auweraert tweeted. “We have seen a few days ago how quickly fire can spread. This is a totally unnecessary tragedy.”

Also voicing concern and calling for urgent refugee help are the aid groups Danish Refugee Council, Doctors of the World and Save the Children.