The World Health Organization (WHO) has received no data from China on new COVID-19 hospitalizations since Beijing ended its zero-COVID policy.
On Thursday, US Secretary of State Antony Blinken urged Beijing to share more information.
Hospitalizations peaked early in December
Beijing decided to ease restrictions on movement on December 7, ending the country's zero-COVID policy.
Hospitalizations peaked at 28,859 through to December 4, which is the highest reported figure in China since the start of the pandemic.
Figures have been absent in the last two reports in December.
The WHO said that it may be that Chinese authorities are struggling to register all cases. "I wouldn't like to say that China is actively not telling us what's going on. I think they're behind the curve," WHO emergencies chief Mike Ryan said.
Reuters said it was told by WHO spokesperson Margaret Harris to direct questions to the country concerned.
Millions of elderly people in China are not fully vaccinated. The country's hospitals have seen a surge in admission of elderly COVID-19 patients in recent weeks.
A paramedic at Chongqing Medical University First Affiliated told the Agence France-Presse (AFP) news agency that he had picked up more than 10 people a day, the vast majority of whom were infected with coronavirus.
"Most of them are elderly people," he told AFP. "A lot of hospital staff are positive as well, but we have no choice but to carry on working."
Washington urges Beijing to share information
US Secretary of State Antony Blinken called on China to share information on its COVID-19 outbreak.
"It is very important for all countries, including China, to focus on people getting vaccinated, making testing and treatment available and, importantly, sharing information with the world about what they're experiencing," Blinken said.
"It has implications not just for China, but for the entire world. So we would like to see that happen."
"We're fully prepared to provide assistance to anyone who asks for it if they think it's useful," Blinken said.
