The WHO reiterated Wednesday the need for China to share more data on its explosive Covid outbreak, while praising Washington's "radical transparency" in its efforts to battle a new sub-variant.
The World Health Organization has repeatedly voiced concern that China's official statistics are not showing the true impact of its current surge in Covid cases.
"WHO still believes that deaths are heavily underreported from China," its emergencies director Michael Ryan told reporters.
He blamed Beijing's narrow definition of what constitutes a Covid death, and also pointed to "the need for doctors in the public health system to be encouraged to report these cases, and not discouraged."
In contrast, he hailed the cooperation of authorities in the United States, where the new Omicron sub-variant XBB.1.5 is spreading rapidly.
"There's been radical transparency on behalf of the United States in terms of engaging with the WHO regarding the data and the impact of that data," he said.
Maria Van Kerkhove, the WHO's technical lead on Covid, said Washington had supplied virtually all the data available so far on XBB.1.5, the most transmissible form of Covid so far.
The sub variant, detected in 38 countries, clearly has a "growth advantage" over other forms of the virus, she told reporters. It is also believed to be better at dodging immunity protections from prior infection or vaccines.
"We don't yet have data on severity," she added.
- World 'cannot close its eyes' -
In China, meanwhile, Ryan warned that despite increased collaboration, "we still do not have adequate information to make a full comprehensive risk assessment."
China abruptly dropped its "zero-Covid" approach last month after three years of enforcing some of the harshest anti-pandemic restrictions in the world. That unleashed a wave of infections that has packed hospitals and overwhelmed crematoriums.
But according to official figures, only 37 Covid-related deaths have been recorded in China since last month out of a population of 1.4 billion.
Faced with such discrepancies, the agency has expressed sympathies with countries that have opted to demand Covid tests from travellers from China.
"In the absence of data, countries have made a decision to take a precautionary approach and (WHO has) said that that is understandable in the circumstances," Ryan said.
Chinese officials however do not appear keen on changing their methodology.
"I don't think it is necessary to look into the cause of death for every case at present," epidemiologist Liang Wannian, head of a government-appointed expert panel, told journalists Wednesday.
"The key task during the pandemic should be treatment."
The UN health agency also expressed concern that China, but also many other countries, were not focusing enough on the testing and sequencing needed to help detect possible new variants of the virus.
Since the peak of the Omicron wave last year, "the number of sequences being shared has dropped by more than 90 percent, and the number of countries sharing sequences has fallen by a third," WHO chief Tedros Adhanom Ghebreyesus told reporters.
Yet as Covid had continued to kill between 10,000 and 14,000 people each week, since September, he said "the world cannot close its eyes and hope this virus will go away.