The World Health Organisation has asked China for details on an increase in respiratory illnesses and reported clusters of pneumonia in children, which its China office calls a "routine" check.

Chinese authorities from the National Health Commission held a media conference on November 13 to report an increase in incidence of respiratory disease.

Authorities attributed the increase to the lifting of COVID-19 restrictions and the circulation of known pathogens such as influenza, Mycoplasma pneumoniae, a common bacterial infection that typically affects younger children, respiratory syncytial virus, and the virus that causes COVID-19.

China and the WHO have faced questions about the transparency of reporting on the earliest COVID-19 cases that emerged in the central Chinese city of Wuhan in late 2019.

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

The WHO said it was not clear if these were associated with an overall increase in respiratory infections previously reported by Chinese authorities or separate events.

The WHO said it had asked for additional epidemiologic and clinical information as well as laboratory results from the reported outbreaks among children, through the International Health Regulations mechanism.

It has also asked China for further information about trends in the circulation of known pathogens and the burden on healthcare systems.

The WHO said it was in contact with clinicians and scientists through its existing technical partnerships and networks in China.

WHO China said it was "routine" to request information on increases in respiratory illnesses and reported clusters of pneumonia in children from member states such as China.

The global agency decided to issue a statement on China to share available information because it received a number of queries about it from media, WHO China said in a statement.

Undiagnosed pneumonia was not mentioned at last week's media conference, according to a transcript, but one speaker said everyone felt as if there had been an increase in respiratory illnesses this year compared with three years ago.

The speaker said global monitoring for Mycoplasma pneumoniae had been at a low during the past three years and epidemics were cyclical, occurring every three to seven years.

Since mid-October, the WHO said northern China had reported an increase in influenza-like illness compared with the same period in the previous three years.

The WHO recommended that people in China follow measures to reduce the risk of respiratory illness.