The World Health Organization asked China for detailed information about a surge in respiratory illnesses in children, including pneumonia, as top pediatric medical centers across the country are overwhelmed with patients.

The WHO cited reports including one this week from ProMED, which tracks outbreaks of infectious diseases around the world, warning of an “undiagnosed pneumonia in children in northern China.” The international health body has requested additional epidemiological and clinical information and test results from Chinese officials, it said in a statement Wednesday.