The removal of most restrictions last month has led to a surge in cases.
But China has stopped publishing daily cases data, and has announced only 22 Covid deaths since December, using its own strict criteria.
"We believe that definition [of a Covid death] is too narrow," WHO emergencies director Dr Michael Ryan said.
Dr Ryan said China's figures "under-represent the true impact of the disease in terms of hospital admissions, in terms of ICU admissions, and particularly in terms of deaths".
BBC reports China last month changed its criteria for what constitutes a Covid death, meaning only those who die of respiratory illnesses are counted.
This goes against WHO guidance, which encourages countries to count the number of excess deaths - how many more people die than would normally be expected based on death figures before the pandemic hit.
Dr Ryan added that China had increased its engagement with the WHO in recent weeks, and said he looked forward to receiving "more comprehensive data."
But he also suggested individual health workers could report their own data and experiences.
"We do not discourage doctors and nurses reporting these deaths and these cases," Dr Ryan said. "We have an open approach to be able to record the actual impact of disease in society."
The UK science data company Airfinity estimates more than two million Covid cases a day in China, and 14,700 deaths.
Since China abandoned key parts of its "zero-Covid" strategy almost a month ago, there have been reports of hospitals and crematoriums being overwhelmed.
More than a dozen nations have introduced travel restrictions on travellers from China. Beijing has criticised these as politically motivated and threatened to retaliate.
On Wednesday, the European Union issued new guidance "strongly" recommending that all member states introduce the requirement that passengers flying from China provide a negative Covid test before their departure.
No new Covid variants have been detected in China, despite the surge in cases. However, the WHO has warned this could be due to a decrease in testing.
The Chinese authorities have announced they are sending medical supplies to rural hospitals before an expected wave of coronavirus infections in the countryside - where vaccination rates are patchy.
Dr Abdi Rahman Mahamud, director of the WHO's alert and response coordination department, has warned China may see another wave of infections as families gather for China's Lunar New Year in a few weeks - one of the country's busiest travel periods.