MANILA (Reuters) - The World Health Organization (WHO) said on Tuesday it was helping Cambodia investigate the cause of an unknown disease that has killed at least 60 children under seven years old, most of whom died within 24 hours of being admitted to hospital.

The children, the majority of whom were younger than three, had been admitted to hospitals in Cambodian capital Phnom Penh and northern city Siem Reap since April due to high fevers and signs of encephalitic or respiratory symptoms, or both, the agency said.

All but one of the 61 children admitted to hospital with the diseases had died with "rapid deterioration of respiratory function," the WHO said.

"At this point in time, the cause and source of the disease in Cambodia is still under investigation," the WHO said in an email to Reuters.

"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 United Nations agency said in a June 30 report that the clinical signs of those afflicted with the disease "appear unusual," with patients suffering from fever and a rapid deterioration of respiratory functions, although platelet counts, liver and renal functions were found normal.

No other hospital patients or staff in Phnom Penh had fallen ill with similar symptoms, the agency's report said.