GENEVA (REUTERS) - The World Health Organisation (WHO) said on Friday (May 27) it had received more than 600 reports of probable cases of acute hepatitis in children, but added that the causes remain unknown and are under investigation.

As at May 26, 650 probable cases have been reported to WHO from 33 countries, with 99 additional cases pending classification, according to the WHO study.

The health authorities around the world are probing a mysterious increase in severe cases of hepatitis - inflammation of the liver - in young children that has resulted in at least nine deaths.

United States health officials said last week that infection with adenovirus, a common childhood virus, is the leading hypothesis for recent cases of severe hepatitis of unknown origin in children.

Hepatitis linked to this type of adenovirus has almost exclusively been associated with immunocompromised children.

The US Centres for Disease Control and Prevention said it was investigating whether Covid-19 infections might be playing a role, as well as other pathogens, medications and risk factors.