The World Health Organization reported Friday that there are at least 650 cases of sudden and unexplained hepatitis in young children under investigation in 33 countries, an increase of about 36 cases over the last international update, about a week ago. Another 99 cases are awaiting classification, CNN reports.

Last week, the European Centre for Disease Prevention and Control reported a total of 614 cases under investigation in 30 countries. About one-third of the cases, 222, are in the UK, and another 216 have been reported in the US, WHO reported Friday.

Most of the children in the international investigation got sick in March and April, according to the update.

Before this outbreak, about half of cases of sudden hepatitis — or inflammation of the liver -- in young children could not be tied to a known cause. But WHO says the cases being investigated now seem to be different from anything doctors have seen before. The hepatitis comes on quickly and appears to be more clinically severe, with a higher proportion of children developing liver failure.

Of a subset of 156 cases with data on hospital admission, about 14% required intensive care, and 12% needed a liver transplant.

WHO says it's not clear whether these are a greater number of cases of sudden hepatitis than doctors would normally expect to see over the same period of time.

Most of the affected children -- 75% -- are young, under the age of 5. Most were healthy before they fell ill.