GENEVA — European countries should stop returning asylum seekers to Bulgaria, because they could face “inhuman and degrading treatment” in Bulgaria’s reception centers, which have been stretched thin by a rising number of arrivals, the United Nations refugee agency said Friday.

Standards in Bulgaria’s reception centers have improved in recent weeks, the Office of the United Nations High Commissioner for Refugees said, but are still “deplorable.” It said it would review the situation in April.

“There were cases where people just weren’t getting food or adequate space to sleep,” said Daniel MacIsaac, a spokesman for the refugee agency, reporting that it had started distributing hot meals at a former army base now serving as a reception center that lacks facilities for cooking or heating. Bulgarian government data showed that nearly half of the asylum seekers returned by other European countries were from Syria, he said.

Austria, Britain, Germany and Switzerland are among 14 European countries that sent 70 asylum seekers back to Bulgaria in the past year under the Dublin system, which provides for claims by migrants seeking entry to Europe to be processed by the first country they arrive in, the refugee agency said.