BERLIN (AP) — The World Health Organization says at least one death has been reported in connection with a mysterious liver disease outbreak affecting children in Europe and the United States.

The U.N. health agency said late Saturday that it has so far received reports of at least 169 cases of “acute hepatitis of unknown origin” from a dozen countries.

The cases were reported in children aged one month to 16 years old, and 17 of those who fell ill required liver transplants. WHO didn’t say in which country the death occurred.

The first cases were recorded in Britain, where 114 children have been sickened.

“It is not yet clear if there has been an increase in hepatitis cases, or an increase in awareness of hepatitis cases that occur at the expected rate but go undetected,” WHO said in a statement.

Experts say the cases may be linked to a virus commonly associated with colds, but further research is ongoing.

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“While adenovirus is a possible hypothesis, investigations are ongoing for the causative agent,” WHO said, noting that the virus has been detected in at least 74 of the cases. At least 20 of the children tested positive for the coronavirus.

WHO said affected countries are stepping up their surveillance of hepatitis cases in children.