Dr. Beat Richner of the Kantha Bopha Children's Hospital in Phnom Penh said he knew of at least 64 cases of the unknown disease and only two children have survived, CNN reported.
"They are hospitalized," he said. "They arrive in our hospital, in the last moments ... they die because their lungs are destroyed."
Richner said he's not sure what is causing the illness.
"I think our idea is an enterovirus or an intoxication of a drug," or both, he said.
Most of the cases appear to be coming from the southern part of the country, though there's no one location that seems to be the source, WHO said.
"WHO supports the Ministry with the investigation and detailed analysis of each of these cases to find out the cause," wrote Dr. Pieter Van Maaren, a representative of WHO in Cambodia, in an e-mail Friday.