The World Health Organization said Tuesday it is assisting Cambodian officials in investigating the mysterious deaths of 61 children shortly after they were admitted to hospitals in the cities of Phnom Penh and Siem Reap since April.

The children, most of whom were under the age of three, were admitted with high fevers and signs of encephalitic or respiratory symptoms, or both.

All but one of the children died with rapid deterioration of respiratory function, the WHO said.

The World Health Organization is working closely with the Cambodia Ministry of Health and other country partners to find out the cause and source of this disease. WHO has offered technical assistance to the government for epidemiology and active case finding, the agency added.