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.