On Wednesday, the WHO said groups including the Programme for Monitoring Emerging Diseases reported clusters of undiagnosed pneumonia in children in northern China.

The alert, based on a report by Taiwanese outlet FTV News, said children's hospitals in Beijing and Liaoning, 500 miles apart, were "overwhelmed with sick children".

"Many, many are hospitalised," Mr Wei, a Beijing citizen, told FTV News. "They don't cough and have no symptoms. They just have a high temperature (fever) and many develop pulmonary nodules."

The WHO said it was unclear if these cases were associated with an overall increase in respiratory infections previously reported by Chinese authorities - or separate events.

The WHO has also asked for further information about trends in the circulation of known pathogens and the burden on healthcare systems. It said it is in contact with clinicians and scientists.

The WHO's office in China on Thursday described the action as a "routine" check.

An increase in incidence of respiratory disease was first disclosed during a news conference last week by Chinese officials from the National Health Commission.

Both China and the WHO have previously faced questions about the transparency of reporting on the earliest COVID cases.

Three years on from when the cases were first detected in the central Chinese city of Wuhan in the latter months of 2019 - there is still debate about the origins of the coronavirus outbreak.