GENEVA, May 10 (AFP) - The WHO said on Tuesday (May 10) that 348 probable cases of hepatitis of unknown origin had been identified, as studies into the potential role of adenovirus and COVID-19 infection gather pace.

The World Health Organization said the leading hypotheses remain those involving adenovirus.

Cases have been reported in 20 countries, with 70 additional cases from a further 13 countries which are pending classification as tests await completion.

Only six countries are reporting more than five cases, with more than 160 being reported in Britain.

"Over the last week, there's been some important progress with the further investigations and some refinements of the working hypotheses," Philippa Easterbrook, from the WHO's global hepatitis programme, told a press conference.

She said Britain had been coordinating a comprehensive set of studies looking at the genetics of the children affected, their immune response, viruses and further epidemiological studies.

The WHO was first informed on Apr 5 of 10 unexplained hepatitis cases in Scotland, detected in children under the age of 10.

The US Centers for Disease Control and Prevention said Friday it was investigating 109 such cases, including five reported deaths.

"At present, the leading hypotheses remain those which involve adenovirus - with also still an important consideration about the role of COVID as well, either as a co-infection or a past infection," Easterbrook said.

Further testing in the past week confirmed that around 70 per cent of the cases tested positive for adenovirus, with sub-type 41 - normally associated with gastroenteritis - the prevalent sub-type, she added.

Testing has also shown that around 18 per cent of cases actively tested positive for COVID-19.

"The big focus over the next week is looking at serological testing for previous exposure and infections with COVID," said Easterbrook.