The World Health Organization (WHO) has asked China for more information on "clusters of undiagnosed pneumonia" reportedly spreading among children in the north of the country.
Non-state media reports say paediatric hospitals in parts of the country are overwhelmed with sick children.
Chinese authorities have attributed a spike in flu-like illnesses this winter to the lifting of Covid measures.
The WHO is urging people in China to take measures to reduce transmission.
In a statement, the UN health agency says it wants more information on reports in the media and from ProMed - a global outbreak surveillance system - of "clusters of undiagnosed pneumonia in children in northern China".
After the WHO statement was released, state-run Xinhua news agency published an article on Thursday which quoted officials of the National Health Commission as saying they were paying close attention to the diagnosis and care of children with respiratory illnesses.
While mentions of China and a wave of infection can get people jittery as it brings memories of the coronavirus pandemic, it's good practice for the WHO to ask for clarity. But until Beijing responds, there is no way of knowing why this spike of infections has come.
Since October, northern China has reported an "increase in influenza-like illness" compared to the same period over the past three years, the WHO adds.
Last week, China's National Health Commission said there had been a rise in several respiratory diseases across the country - in particular influenza, Covid, mycoplasma pneumoniae - a common bacterial infection affecting younger children - and respiratory syncytial virus (RSV).
Officials attributed the rise to the lifting of Covid restrictions.
Other countries, including the UK and the US, saw similar surges in flu-like illnesses once pandemic restrictions were lifted.
"China is likely experiencing a major wave of childhood respiratory infections now as this is the first winter after their lengthy lockdown, which must have drastically reduced the circulation of respiratory bugs, and hence decreased immunity to endemic bugs," said Prof Francois Balloux of the University College of London Genetics Institute.
The WHO says it is unclear if the reported pneumonia outbreak and overall increase in respiratory infections reported by Beijing are linked - and has made an official request for more detailed information.
It has urged people in China to take basic precautions like getting vaccinated, wearing masks and hand-washing.
