"This city (Homs) is really in a desperate situation," UNHCR spokeswoman Melissa Fleming told reporters in Geneva, saying that "being displaced inside Syria is a very scary and a vulnerable situation to be in."
The agency's partner in Syria, the Syrian Arab Red Crescent (SARC), had so far registered a quarter of a million people displaced around the city, she said.
A UNHCR team had made a two-day mission to Homs this week, and had reported that "thousands of displaced people (are) living in unheated communal shelters," Fleming said.
"Half of the city's hospitals are not functional and there are severe shortages of basic supplies, ranging from medicine to blankets, winter clothes and children's shoes," she lamented.
Some of the hospitals had been converted into communal shelters and around 60 percent of the city's doctors had left, Fleming said, adding that many of the city's children had not been able to go to school for the past 18 months.
The UNHCR team had during its two-day mission visited two communal buildings, she said, pointing out that in one of them more than 400 families, or some 2,300 people, were crammed together.
"It's a very, very worrying situation," she said.
Earlier this month, the SARC estimated that at least 2.5 million people were internally displaced in Syria, while UNHCR said Friday the number of refugees registered in surrounding countries had now risen to 465,000.
Jordan now counted 137,998 registered Syrian refugees, Lebanon had 133,349, Turkey 123,747, Iraq 60,307 and North Africa 9,734, the agency said, stressing that tens of thousands of others are believed to have fled Syria but not yet registered for assistance.
Fleming said her agency was especially concerned at reports from refugees arriving in Jordan who say they were specifically targeted en route.
"UNHCR calls on all sides to ensure that civilians have access to safe passage," she said, refusing to say who was believed to be targeting the fleeing refugees.
More than 40,000 people have perished since the violence in Syria began in March 2011, according to the Syrian Observatory for Human Rights.